Mother taking husband into desert for test of endurance (8)
I believe the answer is:
marathon
'test of endurance' is the definition.
(a test of a runner's endurance)
'mother taking husband into desert' is the wordplay.
'mother' becomes 'ma' (Ma is another word for mother or mum).
'taking' means one lot of letters go next to another.
'husband' becomes 'h'.
'into' indicates putting letters inside.
'desert' becomes 'raton' (I've seen this before).
'h' going within 'raton' is 'rathon'.
'ma'+'rathon'='MARATHON'
'for' is the link.
(Other definitions for marathon that I've seen before include "Severe test of endurance" , "Pheidippides' starting point" , "The longest foot-race" , "26-mile foot race" , "The longest foot race in sport" .)