“Very old Tom” superseding name in horse's stall? (10)
I believe the answer is:
equivocate
'stall?' is the definition.
(thesaurus)
'very old tom superseding name in horse's' is the wordplay.
I cannot really see how this works, but
'very' could be 'v' (abbreviation) and 'v' is found within the answer.
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is found in the answer.
'tom' could be 'cat' (a tom is a male cat) and 'cat' is present in the answer.
This explanation may well be incorrect...
(Other definitions for equivocate that I've seen before include "Beat about the bush" , "Use evasive language" .)