Mum and dad touring an isthmus (6)
I believe the answer is:
panama
'isthmus' is the definition.
(I've seen this before)
'mum and dad touring an' is the wordplay.
'mum' becomes 'ma' (ma can mean mother).
'and' is a charade indicator (letters next to each other).
'dad' becomes 'pa' (both can mean one's father).
'touring' is an insertion indicator (touring can mean going round).
'pa' going around 'an' is 'pana'.
'ma' put after 'pana' is 'PANAMA'.
(Other definitions for panama that I've seen before include "country divided by boaters?" , "Great canal connecting Atlantic and Pacific" , "Criticise" , "What may be seen overhead" , "Central American country" .)