Informant keeping head of precinct spellbound (4)
I believe the answer is:
rapt
'spellbound' is the definition.
(thesaurus)
'informant keeping head of precinct' is the wordplay.
'informant' becomes 'rat' (rat is a kind of informant).
'keeping' means one lot of letters goes inside another.
'head of' says to take the initial letters.
The initial letter of 'precinct' is 'p'.
'rat' enclosing 'p' is 'RAPT'.
(Other definitions for rapt that I've seen before include "Fully engaged" , "Held spellbound" , "Delighted" , "Completely absorbed" , "Fascinated in part" .)