Lukewarm's food returned around start of 'Porridge' (5)
I believe the answer is:
tepid
'lukewarm's' is the definition.
('tepid' can be a synonym of 'lukewarm')
'food returned around start of porridge' is the wordplay.
'food' becomes 'diet' (diet is a kind of food**).
'returned' shows that the letters should be reversed in order.
'around' means one lot of letters goes inside another.
'start of' indicates taking the first letters.
The first letter of 'porridge' is 'p'.
'diet' in reverse letter order is 'teid'.
'teid' enclosing 'p' is 'TEPID'.
(Other definitions for tepid that I've seen before include "Of liquid, luke-warm" , "lacking enthusiasm" , "without enthusiasm" , "Unenthusiastic - cool" , "What's a bit warm" .)