Officer, single, seen in county, large (7)
I believe the answer is:
colonel
'officer' is the definition.
(colonel is a kind of officer)
'single seen in county large' is the wordplay.
'single' becomes 'lone' (synonyms).
'seen in' is an insertion indicator.
'county' becomes 'co'.
'large' becomes 'L' (eg in clothes sizes).
'co'+'l'='col'
'lone' inserted into 'col' is 'COLONEL'.
(Other definitions for colonel that I've seen before include "Rank (below brigadier)" , "'No cello for this officer, strangely (7)'" , "Army three downn" , "Senior army officer" , "High-ranking officer" .)