Part of colonel’s unit (3)
I believe the answer is:
one
'unit' is the definition.
('one' can be a synonym of 'unit')
'part of colonel's' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'ONE' can be found hidden inside 'colonels'.
(Other definitions for one that I've seen before include "SECOND PART OF 16" , "1" , "Individual" , "If upper-class, perhaps I" , "Hour after midnight" .)