Start filling in to recycle rubbish (6)
I believe the answer is:
refuse
'rubbish' is the definition.
(thesaurus)
'start filling in to recycle' is the wordplay.
'start' indicates taking the first letters.
'in' means one lot of letters goes inside another.
'to recycle' becomes 'reuse' ('reuse' can be a synonym of 'recycle').
The first letter of 'filling' is 'f'.
'f' placed within 'reuse' is 'REFUSE'.
(Other definitions for refuse that I've seen before include "Waste, garbage" , "Say no to garbage" , "Pass up" , "Trash" , "Decline waste material" .)