Thieves with craving for ultimately breaking into post offices (8)
I believe the answer is:
poachers
'thieves' is the definition.
(poacher is a kind of thief)
'with craving for ultimately breaking into post offices' is the wordplay.
'with' means one lot of letters goes inside another.
'craving' becomes 'ache' (I can't explain this - if you can you should give a lot more credence to this answer).
'ultimately' says to take the final letters.
'breaking into' is an insertion indicator.
'post offices' becomes 'pos' (PO is an abbreviation for post office).
The final letter of 'for' is 'r'.
'r' inserted within 'pos' is 'pors'.
'ache' placed within 'pors' is 'POACHERS'.
Can you help me to learn more?
(Other definitions for poachers that I've seen before include "Illegal takers known to turn gamekeepers" , "Illegal hunters and anglers" .)