Cross section of lengthy bridge (6)
I believe the answer is:
hybrid
'cross' is the definition.
(thesaurus)
'section of lengthy bridge' is the wordplay.
'section of' says the answer is hidden in the clue.
'HYBRID' is hidden in the letters of 'lengthy bridge'.
(Other definitions for hybrid that I've seen before include "Cross breed" , "Mongrel - cross" , "Crossbreed or mixture" , "Of mixed ancestry" , "Crossed offspring" .)