Start to doubt being embraced by unusually genial principal (7)
I believe the answer is:
leading
'principal' is the definition.
(I've seen this before)
'start to doubt being embraced by unusually genial' is the wordplay.
'start to' indicates taking the first letters.
'being embraced by' indicates putting letters inside.
'unusually' is an anagram indicator.
The first letter of 'doubt' is 'd'.
'genial' anagrammed gives 'leaing'.
'd' going within 'leaing' is 'LEADING'.
(Other definitions for leading that I've seen before include "'Showing the way, out in front (7)'" , "Most important; ahead" , "Aligned in front" , "Guiding by going in front" , "In the chair" .)