To have a go after friend's not worth considering (6)
I believe the answer is:
paltry
'not worth considering' is the definition.
(I've seen this before)
'to have a go after friend's' is the wordplay.
'to have' becomes 'try' (trying is a kind of having).
'a go after' says to put letters next to each other.
'friend' becomes 'pal' (pal is a kind of friend).
'try' put after 'pal' is 'PALTRY'.
(Other definitions for paltry that I've seen before include "Meagre; petty, trivial" , "Mean, petty" , "Mean - worthless" , "not worth considering" , "Worthless and trifling" .)