Father turned mean quickly (5)
I believe the answer is:
apace
'quickly' is the definition.
(I know that quickly can be written as apace)
'father turned mean' is the wordplay.
'father' becomes 'pa' (pa can mean dad or father).
'turned' says the letters should be written backwards.
'mean' becomes 'ace' (I can't justify this - if you can you should believe this answer much more).
'pa' reversed gives 'ap'.
'ap'+'ace'='APACE'
Can you help me to learn more?
(Other definitions for apace that I've seen before include "at high speed" , "At a rapid rate" , "flat out" , "What's quickly" , "fast" .)