Active undercover spy recruits British Head of Intelligence (6)
I believe the answer is:
mobile
'active' is the definition.
(thesaurus)
'undercover spy recruits british head of intelligence' is the wordplay.
'undercover spy' becomes 'mole' (mole is a kind of spy. I am not sure about the 'undercover' bit.).
'recruits' indicates putting letters inside.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'head of' says to take the initial letters.
The initial letter of 'intelligence' is 'i'.
'b'+'i'='bi'
'mole' placed around 'bi' is 'MOBILE'.
Can you help me to learn more?
(Other definitions for mobile that I've seen before include "US city" , "hoi polloi, as was" , "Moving" , "For European this is a handy" , "Portable phone" .)