Get along with an environmentalist mostly (5)
I believe the answer is:
agree
'get along with' is the definition.
(thesaurus)
'an environmentalist mostly' is the wordplay.
'an' becomes 'a'.
'environmentalist' becomes 'green' (member of the Green Party).
'mostly' means to remove the last letter (most but not all of the word).
'green' with its last letter taken off is 'gree'.
'a'+'gree'='AGREE'
(Other definitions for agree that I've seen before include "Arrive at a settlement" , "'Assent, concur (5)'" , "Settle" , "Shake on it" , "Have the same opinion" .)