Synthetic polymer unknown in northern half of capital (5)
I believe the answer is:
nylon
'synthetic polymer' is the definition.
(I've seen this before)
'unknown in northern half of capital' is the wordplay.
'unknown' becomes 'y' (algebra).
'in' indicates putting letters inside.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'half of capital' becomes 'lon' (I've seen this in other clues).
'n'+'lon'='nlon'
'y' put inside 'nlon' is 'NYLON'.
(Other definitions for nylon that I've seen before include "Synthetic fabric invented in 1938" , "Synthetic polymer" , "Synthetic material used in textiles" , "Synthetic cloth" , "The first synthetic fibre" .)