Dry river — assistance needed to cross (4)
I believe the answer is:
arid
'dry' is the definition.
(similar in meaning)
'river assistance needed to cross' is the wordplay.
'river' becomes 'r'.
'assistance' becomes 'aid' (synonyms).
'needed to cross' indicates putting letters inside.
'r' put inside 'aid' is 'ARID'.
(Other definitions for arid that I've seen before include "Dry and parched, as with regions" , "water shortage?" , "Drought-hit" , "Tedious; short of rain" , "Sterile" .)