Loves welcoming very quiet colleague (4)
I believe the answer is:
oppo
'colleague' is the definition.
(informal UK term for colleague or friend)
'loves welcoming very quiet' is the wordplay.
'loves' becomes 'oo' (love means 0 in tennis).
'welcoming' is an insertion indicator (in sense of 'inviting in').
'very quiet' becomes 'pp'.
'oo' going around 'pp' is 'OPPO'.
(Other definitions for oppo that I've seen before include "Colleague, friend (colloq.)" , "China" , "Mate" , "Opposite number (colloq.)" , "Counterpart; rival" .)