Rather dull line in food (6)
I believe the answer is:
dryish
'rather dull' is the definition.
(I've seen this before)
'line in food' is the wordplay.
'line' becomes 'ry' (abbreviation for railway).
'in' is an insertion indicator.
'food' becomes 'dish' (dish is a kind of food).
'ry' inserted within 'dish' is 'DRYISH'.
(Other definitions for dryish that I've seen before include "Struggling to stay on the wagon" , "Fairly arid" , "rather dull" , "not very fresh?" , "Rather desiccated" .)