Pile of stones allowing Adam's son to cross river (5)
I believe the answer is:
cairn
'pile of stones' is the definition.
(I've seen this before)
'adam's son to cross river' is the wordplay.
'adam's son' becomes 'cain' (biblical figure).
'to cross' is an insertion indicator.
'river' becomes 'r'.
'cain' enclosing 'r' is 'CAIRN'.
'allowing' is the link.
(Other definitions for cairn that I've seen before include "Stone-pile landmark" , "Stones piled up as a marker" , "Burial mound" , "Terrier - mound of stones" , "Stone mound" .)