Please exercise duck (5)
I believe the answer is:
douse
'duck' is the definition.
(synonyms)
'please exercise' is the wordplay.
'please' becomes 'do' (I've seen this in other clues).
'exercise' becomes 'use' (synonyms).
'do'+'use'='DOUSE'
(Other definitions for douse that I've seen before include "Put out (flame)" , "Throw water over, extinguish" , "Of fire, quench with water" , "snuff" , "Soak" .)