Repellent wound covered by small firm plaster (6)
I believe the answer is:
stucco
'plaster' is the definition.
(I know that stucco is a type of plaster)
'repellent wound covered by small firm' is the wordplay.
'repellent wound' becomes 'tuc' (I can't justify this - if you can you should believe this answer much more).
'covered by' says to put letters next to each other (on top of in a down clue).
'small' becomes 's' (abbreviation - e.g. clothes size).
'firm' becomes 'co' (a firm is a company).
'tuc' put after 's' is 'stuc'.
'stuc'+'co'='STUCCO'
Can you help me to learn more?
(Other definitions for stucco that I've seen before include "Plaster used for moulding and decorating" , "Fine plaster used on wall surfaces" , "Coating material" , "Fine plaster for decoration" , "Fine type of plaster" .)