Indifferent to pressure in unorthodox diet (5)
I believe the answer is:
tepid
'indifferent' is the definition.
(I've seen this in another clue)
'pressure in unorthodox diet' is the wordplay.
'pressure' becomes 'p' (scientific abbreviation).
'in' is an insertion indicator.
'unorthodox' indicates an anagram.
'diet' with letters rearranged gives 'teid'.
'p' inserted into 'teid' is 'TEPID'.
'to' acts as a link.
(Other definitions for tepid that I've seen before include "Showing little interest or enthusiasm" , "Warmish" , "Not very warm (water)" , "Luke warm" , "lacking enthusiasm" .)