Unfortunate fellow lost his head on fewer occasions (7)
I believe the answer is:
hapless
'unfortunate' is the definition.
(similar in meaning)
'fellow lost his head on fewer occasions' is the wordplay.
'fellow' becomes 'chap' (synonyms).
'lost his head' means to remove the first letter.
'on fewer occasions' becomes 'less' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'chap' with its first letter removed is 'hap'.
'hap'+'less'='HAPLESS'
Can you help me to learn more?
(Other definitions for hapless that I've seen before include "Unlucky, unfortunate" , "Ill-starred" , "Ill-fated" , "Piteous" , "Unfortunate, wretched" .)