Most painful infection just starting in a breast (7)
I believe the answer is:
achiest
'most painful' is the definition.
('achy' can be similar in meaning to 'painful')
'infection just starting in a breast' is the wordplay.
'just starting' indicates taking the first letters.
'in' is an insertion indicator.
'breast' becomes 'chest' (I can't explain this - if you can you should give a lot more credence to this answer).
The first letter of 'infection' is 'i'.
'a'+'chest'='achest'
'i' going within 'achest' is 'ACHIEST'.
Can you help me to learn more?
(Other definitions for achiest that I've seen before include "most painful" , "suffering most pain" .)