Fit male visits favourite nation, shortly (5,3)
I believe the answer is:
petit mal
'fit' is the definition.
(petit mal is a kind of fit)
'male visits favourite nation shortly' is the wordplay.
'male' becomes 'm' (common abbreviation).
'visits' means one lot of letters goes inside another (visit can mean to go into).
'favourite' becomes 'pet' (synonyms).
'nation shortly' becomes 'ital' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'pet'+'ital'='petital'
'm' going inside 'petital' is 'PETIT MAL'.
Can you help me to learn more?
(Other definitions for petit mal that I've seen before include "Mild attack of epilepsy (French)" , "Mild epilepsy - metal tip (anag)" .)