Boy with fishing gear, eg lines set on Westminster Bridge (6)
I believe the answer is:
sonnet
'eg lines set on westminster bridge' is the definition.
(Wordsworth's sonnet composed on Westminster Bridge)
'boy with fishing gear' is the wordplay.
'boy' becomes 'son' (synonyms).
'with' is a charade indicator (letters next to each other).
'fishing gear' becomes 'net' (a net is used when fishing).
'son'+'net'='SONNET'
(Other definitions for sonnet that I've seen before include "Fourteen-line 11" , "Poetic fourteen-liner" , "Verse form of 14 lines" , "Shelley's Ozymandias, for example" , "The poem was sent on" .)