Mother's pal developed twitch that was affected by heat (13)
I believe the answer is:
thermoplastic
'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?" .)