They're of little matter when cuddling the cat (5)
I believe the answer is:
atoms
'they're of little matter' is the definition.
The definition suggests an adverb but the answer is not.
'when cuddling the cat' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'cuddling' indicates putting letters inside (inserted letters are embraced or welcomed in).
'the cat' becomes 'tom' (a tom is a male cat).
'as' enclosing 'tom' is 'ATOMS'.
Can you help me to learn more?
(Other definitions for atoms that I've seen before include "Tiny particles" , "Small particles" , "Jots" , "small fragments" , "Minute particles" .)