Extra soft, with a bit of lustre (4)
I believe the answer is:
plus
'extra' is the definition.
(thesaurus)
'soft with a bit of lustre' is the wordplay.
'soft' becomes 'pus' (I am not sure about this - if you are sure you should believe this answer much more).
'with' is an insertion indicator.
'a bit of' suggests taking the first letters.
The initial letter of 'lustre' is 'l'.
'pus' placed around 'l' is 'PLUS'.
Can you help me to learn more?
(Other definitions for plus that I've seen before include "Positive thing" , "Positive and not a negative" , "being a little cross" , "Positive quality" , "Together with; advantage" .)