Hard work securing support for backing mass confection (9)
I believe the answer is:
sweetmeat
'confection' is the definition.
(synonyms)
'hard work securing support for backing mass' is the wordplay.
'hard work' becomes 'sweat' (synonyms).
'securing' indicates putting letters inside.
'support' becomes 'tee' (tee is a kind of support).
'for backing' is a reversal indicator.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'tee' written backwards gives 'eet'.
'eet'+'m'='eetm'
'sweat' enclosing 'eetm' is 'SWEETMEAT'.
(Other definitions for sweetmeat that I've seen before include "Sugary delicacy" , "Sugary confection" , "Confectionery (archaic)" , "Old item of confectionery" , "It may be sucked" .)