Dummy put on crate, briefly (7)
I believe the answer is:
placebo
'dummy' is the definition.
(ineffective or dummy version of a drug)
'put on crate briefly' is the wordplay.
'put' becomes 'place' (synonyms).
'on' is a charade indicator (letters next to each other).
'crate' becomes 'box'.
'briefly' means to remove the last letter.
'box' with its last letter taken away is 'bo'.
'place'+'bo'='PLACEBO'
(Other definitions for placebo that I've seen before include "Imitation pill" , "Something used to calm or humour a person" , "Prescription to soothe" , "Ineffective medication" , "Harmless substance as medicine to sooth patient" .)