Controls drops in sticky substance (5)
I believe the answer is:
resin
'sticky substance' is the definition.
(I've seen this before)
'controls drops in' is the wordplay.
'controls' becomes 'reins in' (to rein in is to control).
'drops' suggests deleting specific letters.
'reinsin' with 'in' removed is 'RESIN'.
(Other definitions for resin that I've seen before include "Glue" , "Sticky substance exuded by pine trees" , "Contributor to plastics industry" , "Rinse out this substance from trees" , "Sticky exudation from trees" .)