Lukewarm fare served up quietly inside (5)
I believe the answer is:
tepid
'lukewarm' is the definition.
(I know that lukewarm can be written as tepid)
'fare served up quietly inside' is the wordplay.
'fare' becomes 'diet' (fare can mean food).
'served up' shows that the letters should be reversed in order (in a down clue, going upwards).
'quietly' becomes 'p' (p for piano and quiet).
'inside' indicates putting letters inside.
'diet' in reverse letter order is 'teid'.
'teid' going around 'p' is 'TEPID'.
(Other definitions for tepid that I've seen before include "Not very enthusiastic" , "Not very warm (water)" , "Warmish" , "What's a bit warm" , "Slightly warm" .)