Powerful alkali not ultimately found in a shop, surprisingly (6)
I believe the answer is:
potash
'powerful alkali' is the definition.
(I've seen this before)
'not ultimately found in a shop surprisingly' is the wordplay.
'ultimately' says to take the final letters.
'found in' is an insertion indicator.
'surprisingly' is an anagram indicator (a surprising ordering of the letters).
The final letter of 'not' is 't'.
'a'+'shop'='ashop'
'ashop' with letters rearranged gives 'poash'.
't' going into 'poash' is 'POTASH'.
(Other definitions for potash that I've seen before include "field dressing" , "Type of fertiliser" , "Alkaline compound" , "Powerful alkali from burnt wood" , "feed in bed?" .)