Sharp fall in pollution?
I believe the answer is:
acid rain
'pollution?' is the definition.
(acid rain is a kind of pollution)
'sharp fall' is the wordplay.
'sharp' becomes 'acid' (I've seen this before).
'fall' becomes 'rain' (raining is a kind of falling**).
'acid'+'rain'='ACID-RAIN'
'in' is the link.
(Other definitions for acid rain that I've seen before include "it's bad for the environment" , "pollution falling" , "that pollutes" , "Form of pollution" , "environmental problem" .)