Help to drink river dry (4)
I believe the answer is:
arid
'dry' is the definition.
(similar in meaning)
'help to drink river' is the wordplay.
'help' becomes 'aid' (synonyms).
'to drink' means one lot of letters goes inside another.
'river' becomes 'r' (abbreviation for river).
'aid' going around 'r' is 'ARID'.
(Other definitions for arid that I've seen before include "Excessively dry" , "Parched and barren" , "Dry raid" , "Dry and parched, as with regions" , "Very dry, having little rain" .)