Fewer than twenty in foreground (10)
I believe the answer is:
underscore
'foreground' is the definition.
'fewer than twenty' is the wordplay.
'fewer than' becomes 'under'.
'twenty' becomes 'score' ('score' can be a synonym of 'twenty').
'under'+'score'='UNDERSCORE'
'in' acts as a link.
(Other definitions for underscore that I've seen before include "It's beneath character to highlight" , "Stress" , "Emphasise" , "19 ?" .)