Pressure to leave friendly colleague (4)
I believe the answer is:
ally
'colleague' is the definition.
(ally can mean a colleague or associate)
'pressure to leave friendly' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'to leave' suggests deleting specific letters.
'friendly' becomes 'pally' (similar in meaning).
'pally' with 'p' taken away is 'ALLY'.
(Other definitions for ally that I've seen before include "Join forces with" , "Join by marriage, treaty etc" , "Team up" , "Friendly nation" , "Accomplice" .)