The sort of mate that can be relied on (6)
I believe the answer is:
stable
'the sort of mate that' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'can be relied on' is the wordplay.
'can' becomes 'able' (I've seen this in other clues).
'be relied' becomes 'st' (I am not sure about this - if you are sure you should believe this answer much more).
'on' is a charade indicator (letters next to each other).
'able' after 'st' is 'STABLE'.
Can you help me to learn more?
(Other definitions for stable that I've seen before include "Animal accommodation is steady" , "On an even keel" , "Permanent" , "Bleats (anag)" , "Where horses are kept" .)