With Clubiona you will not get to the degree of detail needed with a lens - look in the Collins Field Guide on spider of Britain and north-western Europe to see the number of Clubiona and the detail in the adult male palps, which will not be discernible adequately using a lens. With some very few exceptions, it is simply not possible to be sure of an id in this genus without the use of a microscope. A very few species provide clues because they have specific patterns such as Clubiona comta and the much larger C. corticalis, but otherwise you are left with subtle differences in size and sometimes body shape etc as the main guidance in the field - but this cannot be used for more than guesswork identification in the field. Female epigynes can be even more problemmatic, even presenting some difficulty under a microscope on occasions where the details visible don't obviously match the figures in the books. Clubiona are certainly not the genus to choose to begin identification to species level!
In the habitat you describe, there are a number of likely species, C. pallidula, C. terrestris and more, but only a specimen will determine which one.