Feared old fellow on entering (5)
I believe the answer is:
dread
'feared' is the definition.
(thesaurus)
'old fellow on entering' is the wordplay.
'old fellow' becomes 'dad' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'on' becomes 're' (regarding).
'entering' means one lot of letters goes inside another.
'dad' enclosing 're' is 'DREAD'.
Can you help me to learn more?
(Other definitions for dread that I've seen before include "Adder (anag)" , "Affright" , "Fearful expectation" , "Frightful" , "Awe" .)