Small number crossing river, the UK’s longest (6)
I believe the answer is:
severn
'small number crossing river' is the definition.
'Severn' can be an answer for 'river' (River Severn). I am not sure about the 'small number crossing' bit.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for severn that I've seen before include "River flowing into the Bristol Channel" , "Worcester's river" , "A major British river" , "River of England and Wales" , "Welsh and English river" .)