Those with stripes, good in lines (6)
I believe the answer is:
tigers
'those with stripes' is the definition.
(animals with stripes)
'good in lines' is the wordplay.
'good' becomes 'g' (abbreviation).
'in' indicates putting letters inside.
'lines' becomes 'tiers' (tier is a kind of line).
'g' put inside 'tiers' is 'TIGERS'.
(Other definitions for tigers that I've seen before include "strong economies" , "Big striped cats poetically burning bright" , "Large forest felines" , "Large striped felines" , "Great striped animals, poetically burning bright" .)