Mole's holiday by river? (10)
I believe the answer is:
breakwater
'mole's' is the definition.
('breakwater' can be a synonym of 'mole')
'holiday by river?' is the wordplay.
'holiday' becomes 'break' (I've seen this before).
'by' says to put letters next to each other.
'river?' becomes 'water' (river is a kind of water**).
'break'+'water'='BREAKWATER'
(Other definitions for breakwater that I've seen before include "mole" , "Groyne" .)