Money gets plain wool (8)
I believe the answer is:
cashmere
'wool' is the definition.
(cashmere is a kind of wool)
'money gets plain' is the wordplay.
'money' becomes 'cash' (I've seen this before).
'gets' means one lot of letters go next to another.
'plain' becomes 'mere' ('mere' can be similar in meaning to 'plain'**).
'cash'+'mere'='CASHMERE'
(Other definitions for cashmere that I've seen before include "Expensive wool? Search me" , "High-quality wool" , "'Fine, soft wool from a goat (8)'" , "Soft wool of a type of goat" , "Soft fabric made from goats' hair" .)