Gap in river covered by sand? (6)
I believe the answer is:
breach
'gap' is the definition.
(I know that gap can be written as breach)
'river covered by sand?' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'covered by' is an insertion indicator.
'sand?' becomes 'beach' (sand can refer to a sandy beach).
'r' placed within 'beach' is 'BREACH'.
'in' is the link.
(Other definitions for breach that I've seen before include "'Gap, break or hole (6)'" , "Violation of a promise" , "'A custom more 1 down in the ____ ..." , "Break (in a wall, etc)" , "Rupture, rift" .)