Substance found in delicatessen - celery? (7)
I believe the answer is:
essence
'substance' is the definition.
(synonyms)
'in delicatessen celery?' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ESSENCE' is hidden in the letters of 'delicatessen celery'.
'found' is the link.
(Other definitions for essence that I've seen before include "Distinctive inner nature of person or thing" , "Fundamental nature or quality" , "Intrinsic quality" , "Characteristic nature" , "Nature - concentrate" .)