Sentimental type having nervous reaction (8)
I believe the answer is:
romantic
'sentimental' is the definition.
(I've seen this before)
'type having nervous reaction' is the wordplay.
'type' becomes 'roman type' (variety of type eg Times New Roman).
'having' says to put letters next to each other.
'nervous reaction' becomes 'tic' (a nervous tic).
'roman'+'tic'='ROMANTIC'
(Other definitions for romantic that I've seen before include "Soulful or amourous idealist" , "as Darnay perhaps" , "Visionary" , "Sentimental" , "Imaginative or amorous" .)