Consent to old beast of burden turning back (4)
I believe the answer is:
okay
'consent' is the definition.
(thesaurus)
'old beast of burden turning back' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'beast of burden' becomes 'yak' (I've seen this before**).
'turning back' says the letters should be written in reverse.
'yak' in reverse letter order is 'kay'.
'o'+'kay'='OKAY'
'to' is the link.
(Other definitions for okay that I've seen before include "in order" , "Authorise" , "Not terrific but acceptable" , "Agree" , "Satisfactory; permissible" .)