Officer in work unit in the main not losing heart (8)
I believe the answer is:
sergeant
'officer' is the definition.
(sergeant is a kind of officer)
'work unit in the main not losing heart' is the wordplay.
'work unit' becomes 'erg' (erg is a kind of work unit).
'in the' means one lot of letters goes inside another.
'main' becomes 'sea' (the main is an old name for the sea).
'losing heart' says to hollow out the word (remove centre letters).
'not' with its centre taken out is 'nt'.
'erg' inserted inside 'sea' is 'sergea'.
'sergea'+'nt'='SERGEANT'
'in' acts as a link.
(Other definitions for sergeant that I've seen before include "Rank above corporal" , "Police rank" , "Pepper, say" , "officer of the law" , "Copper" .)