Long to see river in grotto (5)
I believe the answer is:
crave
'long' is the definition.
(thesaurus)
'river in grotto' is the wordplay.
'river' becomes 'r' (abbreviation).
'in' indicates putting letters inside.
'grotto' becomes 'cave' (I've seen this before).
'r' going inside 'cave' is 'CRAVE'.
'to see' acts as a link.
(Other definitions for crave that I've seen before include "need, desperately" , "Beg; long for" , "Powerfully desire" , "Hunger for" , "Beseech" .)