Drops not all restraint! (4)
I believe the answer is:
rain
'drops' is the definition.
(drops of water)
'not all restraint' is the wordplay.
'not all' indicates the answer is hidden within the clue (only some of the letters).
'RAIN' is hidden within 'restraint'.
(Other definitions for rain that I've seen before include "Storm" , "Fall in large quantities" , "downfall?" , "Weather - shower" , "Wet weather" .)