Finish off expression in method that’s far from neat (6)
I believe the answer is:
watery
'far from neat' is the definition.
(as in alcohol watered down rather than drunk neat)
'finish off expression in method' is the wordplay.
'finish off' means to remove the last letter.
'expression' becomes 'term' (I've seen this before).
'in' indicates putting letters inside.
'method' becomes 'way' (way of doing something).
'term' with its final letter taken off is 'ter'.
'ter' placed inside 'way' is 'WATERY'.
'that's' acts as a link.
(Other definitions for watery that I've seen before include "Weak, too diluted" , "Aqueous" , "(Of food) thin, tasteless" , "Overly diluted, thin" , "Insipid" .)