Force iron into so long a fabric (7)
I believe the answer is:
taffeta
'a fabric' is the definition.
(taffeta is a kind of fabric)
'force iron into so long' is the wordplay.
'force' becomes 'f' (symbol used in Physics e.g. f=ma).
'iron' becomes 'fe' (Fe is the chemical symbol for iron).
'into' means one lot of letters goes inside another.
'so long' becomes 'tata' (both are ways of saying goodbye).
'f'+'fe'='ffe'
'ffe' inserted inside 'tata' is 'TAFFETA'.
(Other definitions for taffeta that I've seen before include "Shiny silk fabric" , "Thin glossy silk fabric" , "Shiny silk with a crisp texture" , "Shiny fabric for fat fate" , "cloth" .)