Help involving river becoming dry (4)
I believe the answer is:
arid
'dry' is the definition.
('arid' can be similar in meaning to 'dry')
'help involving river' is the wordplay.
'help' becomes 'aid' ('aid' can be a synonym of 'help').
'involving' is an insertion indicator.
'river' becomes 'r'.
'aid' placed around 'r' is 'ARID'.
'becoming' is the link.
(Other definitions for arid that I've seen before include "Dry and infertile" , "Without much rainfall" , "Dry, devoid of interest" , "Parched with heat" , "water shortage?" .)