Dog that is left underneath bridge (7)
I believe the answer is:
spaniel
'dog' is the definition.
(spaniel is a kind of dog)
'that is left underneath bridge' is the wordplay.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'left' becomes 'l' (common abbreviation).
'underneath' is a charade indicator (letters next to each other) (in a down clue, some letters go under others).
'bridge' becomes 'span' (synonyms).
'ie'+'l'='iel'
'iel' put after 'span' is 'SPANIEL'.
(Other definitions for spaniel that I've seen before include "Dog found among the alpines" , "See 13" , "Gundog; obsequious type" , "Kind of dog, may be a cocker" , "See 1 Across" .)