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' indicates one should take the final letters.
'found in' is an insertion indicator.
'surprisingly' indicates an anagram (a surprising ordering of the letters).
The last letter of 'not' is 't'.
'a'+'shop'='ashop'
'ashop' with letters rearranged gives 'poash'.
't' placed 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?" .)