Colleague in middle of room keeping very quiet (4)
I believe the answer is:
oppo
'colleague' is the definition.
(informal UK term for colleague or friend)
'middle of room keeping very quiet' is the wordplay.
'middle of' indicates the central letters.
'keeping' means one lot of letters goes inside another.
'very quiet' becomes 'pp'.
The middle letters of 'room' are 'oo'.
'oo' going around 'pp' is 'OPPO'.
'in' is the link.
(Other definitions for oppo that I've seen before include "China" , "Counterpart; rival" , "Mate" , "Colleague, friend (colloq.)" , "Opposite number (colloq.)" .)