Agent comes back home to Spain to explain (6)
I believe the answer is:
define
'explain' is the definition.
(thesaurus)
'agent comes back home to spain' is the wordplay.
'agent' becomes 'fed' (fed is a kind of agent**).
'comes back' is a reversal indicator.
'home' becomes 'in' ('he's home' can mean 'he's in').
'to spain' becomes 'e' (abbreviation for Espana).
'fed' in reverse letter order is 'def'.
'def'+'in'+'e'='DEFINE'
'to' acts as a link.
(Other definitions for define that I've seen before include "State the meaning of" , "Fix the meaning of" , "Give exact meaning as of a word" , "Explain the meaning of a word" , "Characterise" .)