Toast is lightly brown across length (6)
I believe the answer is:
salute
'toast' is the definition.
(synonyms)
'lightly brown across length' is the wordplay.
'lightly brown' becomes 'saute' (to saute is to fry or brown quickly).
'across' is an insertion indicator.
'length' becomes 'l' (abbreviation in geometry).
'saute' placed around 'l' is 'SALUTE'.
'is' is the link.
(Other definitions for salute that I've seen before include "Hail with a gesture" , "A greeting, often military" , "Ceremonial firing of guns" , "Honour" , "Acknowledge (superior)" .)