Retained most of key part (4)
I believe the answer is:
kept
'retained' is the definition.
(keeping is a kind of retaining)
'most of key part' is the wordplay.
'most of' means to remove the last letter.
'part' becomes 'pt' (abbreviation).
'key' with its last letter removed is 'ke'.
'ke'+'pt'='KEPT'
(Other definitions for kept that I've seen before include "Retained possession" , "Saved" , "Looked after" , "Maintained, detained" , "Retained, held back" .)