Crossword Genius

So Eric frantically pulls in stomach when using one? (9)

Ross

I believe the answer is:

costumier

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'using one?' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.

'so eric frantically pulls in stomach' is the wordplay.
'frantically' indicates anagramming the letters.
'pulls in' indicates putting letters inside.
'stomach' becomes 'tum' (synonyms).
'so'+'eric'='soeric'
'soeric' is an anagram of 'cosier'.
'cosier' placed around 'tum' is 'COSTUMIER'.

'when' is the link.

Can you help me to learn more?

(Other definitions for costumier that I've seen before include "Tom Cruise (anag)" , "Supplier of stage clothes for actors" , "theatrical dresser" , "fashioned by me" , "The person that dressed" .)

I've seen this clue in The Guardian.
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