Fast river receding — see crossing (6)
I believe the answer is:
speedy
'fast' is the definition.
(similar in meaning)
'river receding see crossing' is the wordplay.
'river' becomes 'Dee' (River Dee).
'receding' shows that the letters should be reversed in order.
'see' becomes 'spy' (spy can mean to see or observe).
'crossing' is an insertion indicator.
'dee' in reverse letter order is 'eed'.
'eed' placed into 'spy' is 'SPEEDY'.
(Other definitions for speedy that I've seen before include "Swift" , "Expeditious" , "Moving quickly" , "Fast-moving" , "Express" .)