Endless space for sitting down with Rex - a hunting dog (6)
I believe the answer is:
setter
'hunting dog' is the definition.
(I know that setter is a type of gun dog)
'endless space for sitting down with rex a' is the wordplay.
I cannot really see how this works, but
'down' could be 'set' (similar in meaning) and 'set' is present in the answer.
'rex' could be 'r' (abbreviation e.g. GR=George Rex, King George) and 'r' is found in the answer.
The remaining letters 'te' 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).
Can you help me to learn more?
(Other definitions for setter that I've seen before include "Puzzle composer" , "Thematic type [DOG]" , "Dog found in street" , "Canine" , "I may pose a problem" .)