Leave countless cakes I see to bravely diet, say (5,10)
I believe the answer is:
split infinitive
'to bravely diet say' is the definition.
('to bravely diet' splits the infinitive 'to diet')
'leave countless cakes i see' is the wordplay.
'leave' becomes 'split' (I've seen this before).
'countless' becomes 'infinite' (synonyms).
'cakes' indicates putting letters inside (cake can mean to cover).
'see' becomes 'v' (Latin 'vide', used to direct a reader to a source).
'i'+'v'='iv'
'infinite' placed around 'iv' is 'infinitive'.
'split'+'infinitive'='SPLIT INFINITIVE'
(Other definitions for split infinitive that I've seen before include "Grammatical error to many" , "structure some object to" , "To boldly go" .)