Crossword Genius

Mother's pal developed twitch that was affected by heat (13)

Ross

I believe the answer is:

thermoplastic

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'affected by heat' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.

'mother's pal developed twitch' is the wordplay.
'developed' indicates anagramming the letters.
'twitch' becomes 'tic' (tic is a kind of twitch).
'mothers'+'pal'='motherspal'
'motherspal' with letters rearranged gives 'thermoplas'.
'thermoplas'+'tic'='THERMOPLASTIC'

'that was' is the link.

Can you help me to learn more?

(Other definitions for thermoplastic that I've seen before include "Type of resin" , "PET?" .)

I've seen this clue in The Daily Express.
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