Help needed to cross river that's dry (4)
I believe the answer is:
arid
'dry' is the definition.
(similar in meaning)
'help needed to cross river' is the wordplay.
'help' becomes 'aid' (synonyms).
'needed to cross' indicates putting letters inside.
'river' becomes 'r' (abbreviation for river).
'aid' placed around 'r' is 'ARID'.
'that's' acts as a link.
(Other definitions for arid that I've seen before include "(Of climate) dry" , "Lacking sufficient rainfall" , "Dry, devoid of interest" , "Drought-hit" , "water shortage?" .)