Slope used by popular Conservative before row (7)
I believe the answer is:
incline
'slope' is the definition.
(I know that slope can be written as incline)
'popular conservative before row' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'conservative' becomes 'C' (UK political abbreviation).
'before' is a charade indicator (letters next to each other).
'row' becomes 'line' (as in a row of things).
'in'+'c'+'line'='INCLINE'
'used by' is the link.
(Other definitions for incline that I've seen before include "Bend forwards" , "Lean, slope" , "Tend - slope" , "tip" , "Gradient" .)