Fall off in spill (5)
I believe the answer is:
taper
I believe this clue is a double definition.
'fall off' is the first definition.
(I've seen this before)
'spill' is the second definition.
(thesaurus)
'in' is the link.
(Other definitions for taper that I've seen before include "Become narrow" , "Diminish gradually" , "Wind down" , "Get thinner towards the end" , "He records" .)