Dad backing expert without delay (5)
I believe the answer is:
apace
'without delay' is the definition.
(I've seen this in another clue)
'dad backing expert' is the wordplay.
'dad' becomes 'pa' (both can mean one's father).
'backing' says the letters should be written in reverse.
'expert' becomes 'ace' (ace can informally mean very good).
'pa' back-to-front is 'ap'.
'ap'+'ace'='APACE'
(Other definitions for apace that I've seen before include "What's quickly" , "at high speed" , "At a rapid rate" , "Swiftly, quickly" , "Quick" .)