Alert old retainer saving bishop (9)
I believe the answer is:
observant
'alert' is the definition.
(thesaurus)
'old retainer saving bishop' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'retainer' becomes 'servant' ('servant' can be a synonym of 'retainer'**).
'saving' indicates putting letters inside.
'bishop' becomes 'b' (abbreviation used in chess).
'o'+'servant'='oservant'
'oservant' enclosing 'b' is 'OBSERVANT'.
(Other definitions for observant that I've seen before include "taking things in" , "diligent in duty" , "Closely watching" , "Quick at noticing things, attentive" , "keeping close eye on things" .)