Pastry with problematic calories? Not so! (6)
I believe the answer is:
eclair
'pastry' is the definition.
(I know that eclair is a type of pastry)
'problematic calories? not so' is the wordplay.
'problematic' is an anagram indicator.
'not' suggests deleting specific letters.
'calories' with 'so' taken out is 'calrie'.
'calrie' is an anagram of 'ECLAIR'.
'with' acts as a link.
(Other definitions for eclair that I've seen before include "Lacier (anag.)" , "Bakery item" , "cream-cake" , "A long thin cake with chocolate covering" , "Finger-shaped cake with cream filling" .)