Beneath going after small part (6)
I believe the answer is:
sunder
'part' is the definition.
(thesaurus)
'beneath going after small' is the wordplay.
'beneath' becomes 'under' (I've seen this before).
'going after' is a charade indicator (letters next to each other).
'small' becomes 's' (abbreviation - e.g. clothes size).
'under' put after 's' is 'SUNDER'.
(Other definitions for sunder that I've seen before include "Red sun could part" , "Tear apart, sever" , "Nursed so as to tear apart" , "Separate the odd red sun" , "Cut" .)