US runner returned old cap and waterproof (7)
I believe the answer is:
potomac
'us runner' is the definition.
(US river - something that 'runs')
'returned old cap and waterproof' is the wordplay.
'returned' says the letters should be written backwards.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'cap' becomes 'top' (synonyms).
'and' is a charade indicator (letters next to each other).
'waterproof' becomes 'mac' (mac is a kind of waterproof).
'o'+'top'='otop'
'otop' backwards is 'poto'.
'poto'+'mac'='POTOMAC'
(Other definitions for potomac that I've seen before include "to which banks in Washington have access" , "River of Washington DC" , "Capital outflow" , "Washington's river" , "Water flowing" .)