Army officer, single, entering county, large (7)
I believe the answer is:
colonel
'army officer' is the definition.
(I've seen this before)
'single entering county large' is the wordplay.
'single' becomes 'lone' (synonyms).
'entering' means one lot of letters goes inside another.
'county' becomes 'co'.
'large' becomes 'L' (eg in clothes sizes).
'co'+'l'='col'
'lone' going into 'col' is 'COLONEL'.
(Other definitions for colonel that I've seen before include "Comissioned military officer" , "High-ranking officer" , "Army rank" , "Senior army officer" , "Commander" .)