Fish cross river in UK, lying low (7)
I believe the answer is:
lurking
'lying low' is the definition.
'fish cross river in uk' is the wordplay.
'fish' becomes 'ling' (ling is a kind of fish**).
'cross' is an insertion indicator.
'river' becomes 'r'.
'in' indicates putting letters inside.
'r' inserted within 'uk' is 'urk'.
'ling' placed around 'urk' is 'LURKING'.
(Other definitions for lurking that I've seen before include "Lying in wait, to ambush say" , "Skulking" , "Lying hidden in ambush" , "Waiting in ambush" , "In hiding for ambush" .)