Good shower? It may swell growth of this (5)
I believe the answer is:
grain
'it may swell growth of this' is the definition.
The definition suggests a singular noun which matches the answer.
'good shower?' is the wordplay.
'good' becomes 'g' (abbreviation).
'shower?' becomes 'rain' (shower is a kind of rain).
'g'+'rain'='GRAIN'
(Other definitions for grain that I've seen before include "Small particle - weight" , "Single piece of sand etc" , "Small hard particle, of rice perhaps" , "bit of corn" , "basis for whisky" .)