Father and son, second to qualify (4)
I believe the answer is:
pass
'qualify' is the definition.
(as in passing an exam or qualification)
'father and son second' is the wordplay.
I cannot really see how this works, but
'father' could be 'pa' (pa can mean dad or father) and 'pa' is found within the answer.
'son' could be 'ss' (I've seen this in other clues) and 'ss' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'to' is the link.
Can you help me to learn more?
(Other definitions for pass that I've seen before include "Exam success" , "Cross-mountain route; permit" , "Blow over" , "Eg, Khyber, Brenner" , "Overtake; mountain way" .)