Hawk-eyed old British man (9)
I believe the answer is:
observant
'hawk-eyed' is the definition.
(thesaurus)
'old british man' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'man' becomes 'servant' (man can mean someone's personal servant).
'o'+'b'+'servant'='OBSERVANT'
(Other definitions for observant that I've seen before include "keeping close eye on things" , "Perceptive" , "paying close attention" , "Watchful" , "Quick at noticing things, attentive" .)