Intelligence unit in special forces (3)
I believe the answer is:
cia
'intelligence unit' is the definition.
(Central Intelligence Agency)
'in special forces' is the wordplay.
'in' indicates the answer is hidden within the clue.
'CIA' can be found hidden inside 'special forces'.
(Other definitions for cia that I've seen before include "Famous American intelligence agency, advises President" , "American intelligence organisation" , "Famous intelligence agency in US" , "American secret service agency" , "American spies." .)