Dad turned expert at a rate of knots (5)
I believe the answer is:
apace
'at a rate of knots' is the definition.
(both can mean very fast)
'dad turned expert' is the wordplay.
'dad' becomes 'pa' (synonyms).
'turned' is a reversal indicator.
'expert' becomes 'ace' (ace is a kind of expert).
'pa' written backwards gives 'ap'.
'ap'+'ace'='APACE'
(Other definitions for apace that I've seen before include "Quick" , "fast" , "At a rapid rate" , "flat out" , "(Move on) swiftly" .)