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