Peter and Tilly's outside wedding is light (6,5)
I believe the answer is:
safety match
'is light' is the definition.
(I've seen this before)
'peter and tilly's outside wedding' is the wordplay.
'peter' becomes 'safe' (slang term).
'and' says to put letters next to each other.
'outside' means to remove the middle letters.
'wedding' becomes 'match' (match can mean a marriage).
'tilly' with its centre taken out is 'ty'.
'safe'+'ty'+'match'='SAFETY-MATCH'
(Other definitions for safety match that I've seen before include "light" , "It needs a special surface to ignite" , "limited striker" , "Home striker" , "Not a dangerous striker" .)