Obscure diet once developed by head of research (9)
I believe the answer is:
recondite
'obscure' is the definition.
(synonyms)
'diet once developed by head of research' is the wordplay.
'developed' indicates an anagram.
'by' means one lot of letters go next to another.
'head of' says to take the initial letters.
The initial letter of 'research' is 'r'.
'diet'+'once'='dietonce'
'dietonce' anagrammed gives 'econdite'.
'econdite' put after 'r' is 'RECONDITE'.
(Other definitions for recondite that I've seen before include "(Of knowledge) obscure" , "Difficult to understand" , "Mysterious" , "not widely known" , "Direct one to be obscure or difficult" .)