Stand firm when second son leaves for exam (5)
I believe the answer is:
resit
'for exam' is the definition.
(as in resitting an exam)
'stand firm when second son' is the wordplay.
I cannot really understand how this works, but
'second' could be 're' (prefix meaning second in eg review, re-education) and 're' is found within the answer.
'son' could be 's' (genealogical abbreviation for son) and 's' is located in the answer.
The remaining letters 'it' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'leaves' acts as a link.
Can you help me to learn more?
(Other definitions for resit that I've seen before include "Student's test" , "Take examination again" , "later exam" , "second exam" , "Repeat (an exam)" .)