Smaller bit in cake breaking hearts, perhaps? (7)

I believe the answer is:

subunit

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'smaller bit' is the definition.
The definition suggests a singular noun which matches the answer.

'cake breaking hearts perhaps?' is the wordplay.
'cake' becomes 'bun' (I've seen this before).
'breaking' indicates putting letters inside.
'hearts perhaps?' becomes 'suit' (I've seen this in other clues).
'bun' placed within 'suit' is 'SUBUNIT'.

'in' is the link.

Can you help me to learn more?

(Other definitions for subunit that I've seen before include "Distinct component of something bigger" , "Constituent" .)

I've seen this clue in The Telegraph.

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