Clothing that’s rubbish, lacking maturity (4)
I believe the answer is:
garb
'clothing' is the definition.
(garb is a kind of clothing)
'rubbish lacking maturity' is the wordplay.
'rubbish' becomes 'garbage' (both can mean refuse or trash).
'lacking' is a deletion indicator.
'maturity' becomes 'age' (I've seen this before).
'garbage' with 'age' removed is 'GARB'.
'that's' acts as a link.
(Other definitions for garb that I've seen before include "Clothes to brag about" , "Dress - clothing" , "Style of clothing" , "Attire, clothing" , "Kit" .)