Spill drop of Ribena on cassette (5)
I believe the answer is:
taper
'spill' is the definition.
(thesaurus)
'drop of ribena on cassette' is the wordplay.
'drop of' suggests taking the first letters.
'on' is a charade indicator (letters next to each other).
'cassette' becomes 'tape' (I've seen this before).
The initial letter of 'ribena' is 'r'.
'r' put after 'tape' is 'TAPER'.
(Other definitions for taper that I've seen before include "Convex shape that narrows toward a point" , "Narrow to a point" , "Recorder" , "Become thinner - lighter" , "Prate (anag.)" .)