Quiet colleague, friendly (5)
I believe the answer is:
pally
'friendly' is the definition.
('pally' can be similar in meaning to 'friendly')
'quiet colleague' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano').
'colleague' becomes 'ally' (I've seen this before).
'p'+'ally'='PALLY'
(Other definitions for pally that I've seen before include "Close, friendly" , "Very familiar" , "Thick" , "On friendly terms" , "Chummy" .)