Tail to catch old spy (5)
I believe the answer is:
scout
'spy' is the definition.
(thesaurus)
'tail to catch old' is the wordplay.
'tail' becomes 'scut' (scut is a kind of tail**).
'to catch' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'scut' going around 'o' is 'SCOUT'.
(Other definitions for scout that I've seen before include "Talent seeker - boy on a jamboree" , "Person on the lookout" , "One searches for and recruits talented people" , "Soldier on a recce mission" , "One on a recce mission" .)