Skirting river, reached cave (6)
I believe the answer is:
grotto
'cave' is the definition.
(I've seen this before)
'skirting river reached' is the wordplay.
'skirting' is an insertion indicator.
'river' becomes 'r'.
'reached' becomes 'gotto' (getting to is a kind of reaching).
'r' put within 'gotto' is 'GROTTO'.
(Other definitions for grotto that I've seen before include "Attractive natural, or artificial, cave" , "(Imitation) cave" , "It's left on Dec 24 by Santa and his reindeer" , "Artificially or naturally ornamented cave" , "Type of cave, often with religious significance" .)