Indifferent to pressure in new diet (5)
I believe the answer is:
tepid
'indifferent' is the definition.
(I've seen this in another clue)
'pressure in new diet' is the wordplay.
'pressure' becomes 'p' (scientific abbreviation).
'in' is an insertion indicator.
'new' indicates an anagram.
'diet' anagrammed gives 'teid'.
'p' inserted into 'teid' is 'TEPID'.
'to' is the link.
(Other definitions for tepid that I've seen before include "without passion" , "Luke warm" , "Slightly warm" , "Not very warm (water)" , "Of liquid, luke-warm" .)