Light cattle eating primarily dandelion leaves (6)
I believe the answer is:
kindle
'light' is the definition.
(I've seen this before)
'cattle eating primarily dandelion leaves' is the wordplay.
'cattle' becomes 'kine' (synonyms).
'eating' indicates putting letters inside.
'primarily' says to take the initial letters.
The initial letters of 'dandelion leaves' is 'dl'.
'kine' enclosing 'dl' is 'KINDLE'.
(Other definitions for kindle that I've seen before include "Inflame" , "Ignite" , "Rouse" , "Set of fire" , "reading facilitated by this?" .)