Lukewarm food served up, including pathetic starter (5)
I believe the answer is:
'lukewarm' is the definition.
(I know that lukewarm can be written as tepid)
'food served up including pathetic starter' is the wordplay.
'food' becomes 'diet' (diet is a kind of food).
'served up' says the letters should be written backwards (in a down clue, going upwards).
'including' means one lot of letters goes inside another.
'starter' suggests taking the first letters.
The initial letter of 'pathetic' is 'p'.
'diet' backwards is 'teid'.
'teid' going around 'p' is 'TEPID'.
(Other definitions for tepid that I've seen before include "Slightly warm" , "without passion" , "lacking enthusiasm" , "without enthusiasm" , "Unenthusiastic - cool" .)