No wife to advise aged headmaster (6)
I believe the answer is:
arnold
'headmaster' is the definition.
(I've seen this before)
'no wife to advise aged' is the wordplay.
'no' indicates named letters should be taken away.
'wife' becomes 'w' (abbreviation).
'to advise' becomes 'warn' (warning is a kind of advising).
'aged' becomes 'old' (similar in meaning).
'warn' with 'w' taken away is 'arn'.
'arn'+'old'='ARNOLD'
(Other definitions for arnold that I've seen before include "Another poet" , "Essayist" , "Victorian poet" , "Sir Malcolm -, composer (Bridge over the River Kwai)" , "Sir Malcolm -, English composer" .)