Pain in the arse stealing foremost of Tesco's bread (5)
I believe the answer is:
pitta
'bread' is the definition.
(I've seen this before)
'pain in the arse stealing foremost of tesco's' is the wordplay.
'pain in the arse' becomes 'pita' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'stealing' means one lot of letters goes inside another.
'foremost of' suggests taking the first letters.
The initial letter of 'tescos' is 't'.
'pita' enclosing 't' is 'PITTA'.
Can you help me to learn more?
(Other definitions for pitta that I've seen before include "Flat bread that's split to take a filling" , "Type of flat bread that's split to hold a filling" , "Slightly leavened bread" , "Flat hollow unleavened bread" , "Middle Eastern bread" .)