Paints, say, old-fashioned parent, one too short (9)
I believe the answer is:
materials
'paints say' is the definition.
(paints are materials used in art)
'old-fashioned parent one too short' is the wordplay.
'old-fashioned parent' becomes 'mater' (old-fashioned term for 'mother').
'one' becomes 'i' (Roman numeral).
'too' becomes 'also' (synonyms).
'short' means to remove the last letter.
'also' with its last letter taken away is 'als'.
'mater'+'i'+'als'='MATERIALS'
(Other definitions for materials that I've seen before include "Substances that go into the making something" , "textiles" , "See 10" , "Items needed to tackle a job" .)