Help crossing river that's dry (4)
I believe the answer is:
arid
'dry' is the definition.
(similar in meaning)
'help crossing river' is the wordplay.
'help' becomes 'aid' (synonyms).
'crossing' means one lot of letters goes inside another (to cross can mean to go over or around).
'river' becomes 'r' (abbreviation for river).
'aid' going around 'r' is 'ARID'.
'that's' is the link.
(Other definitions for arid that I've seen before include "Dry and unfertile" , "Without much rainfall" , "Rainless (region)" , "Sterile" , "Drought-hit" .)