A firm in High Street ultimately closes, no matter what (2,3,5)
I believe the answer is:
at all costs
'no matter what' is the definition.
(I've seen this in another clue)
'a firm in high street ultimately closes' is the wordplay.
'a firm' becomes 'co' (a firm is a company).
'in' means one lot of letters goes inside another.
'high' becomes 'atall' (I am not sure about this - if you are sure you should believe this answer much more).
'street' becomes 'st' (abbreviation).
'ultimately' says to take the final letters.
The final letter of 'closes' is 's'.
'atall'+'st'='atallst'
'co' put inside 'atallst' is 'atallcost'.
'atallcost'+'s'='AT ALL COSTS'
(Other definitions for at all costs that I've seen before include "whatever may happen" , "Regardless of the price" , "no matter what the consequences may be" , "come what may" , "Regardless of expense" .)