Glittering bridge above lake filling sides of gully (7)
I believe the answer is:
spangly
'glittering' is the definition.
(I've seen this in another clue)
'bridge above lake filling sides of gully' is the wordplay.
'bridge' becomes 'span' (synonyms).
'above' is a charade indicator (letters next to each other) (in a down clue, some letters go above others).
'lake' becomes 'L' (geographical abbreviation).
'filling' is an insertion indicator.
'sides of' suggests removing the centre (only the letters on the sides remain).
'gully' with its middle taken out is 'gy'.
'l' placed inside 'gy' is 'gly'.
'span'+'gly'='SPANGLY'
(Another definition for spangly that I've seen is " Covered with jewels, sequins etc".)