Delighted to hold sealant, not initially stretchy (11)
I believe the answer is:
elasticated
'stretchy' is the definition.
(I've seen this before)
'delighted to hold sealant not initially' is the wordplay.
'delighted' becomes 'elated' (I've seen this before).
'to hold' indicates putting letters inside.
'sealant' becomes 'mastic' (I've seen this before).
'not initially' means to remove the first letter.
'mastic' with its first letter taken away is 'astic'.
'elated' enclosing 'astic' is 'ELASTICATED'.
(Other definitions for elasticated that I've seen before include "Stretchy" , "As knickers may be" .)