Unite worried following argument against firm (11)
I believe the answer is:
consolidate
'unite' is the definition.
(I know that consolidate is a kind of the unite)
'worried following argument against firm' is the wordplay.
'worried' becomes 'ate' (something worrying may eat someone up).
'following' is a charade indicator (letters next to each other).
'argument against' becomes 'con' (as in pros and cons).
'firm' becomes 'solid' (synonyms).
'con'+'solid'='consolid'
'ate' after 'consolid' is 'CONSOLIDATE'.
(Other definitions for consolidate that I've seen before include "Unify" , "Fuse" , "Make stronger; combine into one" , "Join together - strengthen" , "Combine into a connected whole" .)