Agent in his pyjamas (3)
I believe the answer is:
spy
'agent' is the definition.
(I know that spy is a type of intelligence agent)
'in his pyjamas' is the wordplay.
'in' indicates the answer is hidden within the clue.
'SPY' is hidden in the letters of 'his pyjamas'.
(Other definitions for spy that I've seen before include "Secret agent of government" , "Glimpse" , "Keep watch secretly, maybe for foreign government" , "Undercover agent" , "Catch sight of" .)