Shot in the arm in Washington, I collapsed (5)
I believe the answer is:
tonic
'shot in the arm' is the definition.
(both can mean something which stimulates)
'in washington i collapsed' is the wordplay.
'in' says the answer is hidden in the clue.
'TONIC' is hidden within 'washington i collapsed'.
(Other definitions for tonic that I've seen before include "Invigorating medicine - keynote" , "medicinal preparation" , "It goes with gin" , "Quinine drink; keynote" , "A pick-me-up drink, nice with gin" .)