Officer in a unit, leanest, not as corrupt (10)
I believe the answer is:
lieutenant
'officer' is the definition.
(lieutenant is a kind of officer)
'a unit leanest not as corrupt' is the wordplay.
'not' indicates named letters should be taken away.
'corrupt' indicates anagramming the letters.
'a'+'unit'+'leanest'='aunitleanest'
'aunitleanest' with 'as' removed is 'unitleanet'.
'unitleanet' anagrammed gives 'LIEUTENANT'.
'in' acts as a link.
(Other definitions for lieutenant that I've seen before include "Army officer" , "UK rank below captain" , "Army rank" , "Deputy" , "Commissioned officer" .)