Son gets tot up, interrupting rest (6)
I believe the answer is:
laddie
'son' is the definition.
(thesaurus)
'tot up interrupting rest' is the wordplay.
'tot up' becomes 'add' ('add' can be a synonym of 'tot up').
'interrupting' means one lot of letters goes inside another.
'rest' becomes 'lie' (lying is a kind of resting).
'add' going into 'lie' is 'LADDIE'.
'gets' acts as a link.
(Other definitions for laddie that I've seen before include "Sonny" , "Youngster" , "Child" , "(Scots) boy" , "Scottish boy" .)