Make carefree sound when crossing street (7)
I believe the answer is:
whistle
'make carefree sound' is the definition.
(someone without a care in the world might whistle)
'when crossing street' is the wordplay.
'when' becomes 'while'.
'crossing' is an insertion indicator (to cross can mean to go over or around).
'street' becomes 'st' (abbreviation).
'while' going around 'st' is 'WHISTLE'.
(Other definitions for whistle that I've seen before include "Hits Lew to make shrill sound" , "Pucker up and blow" , "Instrument that's sometimes piercing" , "For something, ask in vain" , "Instrument; suit (slang)" .)