Join in with objective, reversing committee's conclusion (9)
I believe the answer is:
integrate
'join' is the definition.
(I've seen this before)
'in with objective reversing committee's conclusion' is the wordplay.
'with' is a charade indicator (letters next to each other).
'objective' becomes 'target' (synonyms).
'reversing' says the letters should be written backwards.
'conclusion' says to take the final letters.
The final letter of 'committee' is 'e'.
'target' back-to-front is 'tegrat'.
'in'+'tegrat'+'e'='INTEGRATE'
(Other definitions for integrate that I've seen before include "Incorporate into the main group" , "Get together" , "Join" , "Combine into a whole, unite" , "Mix" .)