Principal worry involves alien backing resistance (4,7)
I believe the answer is:
head teacher
'principal' is the definition.
(synonyms)
'worry involves alien backing resistance' is the wordplay.
'worry' becomes 'headache' (synonyms).
'involves' is an insertion indicator.
'alien' becomes 'et' (extra terrestrial).
'backing' shows that the letters should be reversed in order.
'resistance' becomes 'r' (electronics abbreviation).
'et' reversed gives 'te'.
'headache' placed around 'te' is 'headteache'.
'headteache'+'r'='HEAD TEACHER'
(Another definition for head teacher that I've seen is " School principal".)