Gap in a lively river (8)
I believe the answer is:
aperture
'gap' is the definition.
(aperture is a kind of gap)
'a lively river' is the wordplay.
'lively' becomes 'pert' (I've seen this in another clue).
'river' becomes 'ure' (River Ure).
'a'+'pert'+'ure'='APERTURE'
'in' acts as a link.
(Other definitions for aperture that I've seen before include "Opening (for a photographer?)" , "Opening, gap" , "Narrow opening" , "An opening that admits light" , "Opening, to let light enter a camera perhaps" .)