Helping to hold bay's head in stable (7)
I believe the answer is:
abiding
'stable' is the definition.
(thesaurus)
'helping to hold bay's head' is the wordplay.
'helping' becomes 'aiding' ('aid' can be a synonym of 'help'**).
'to hold' means one lot of letters goes inside another.
'head' suggests taking the first letters.
The initial letter of 'bays' is 'b'.
'aiding' placed around 'b' is 'ABIDING'.
'in' acts as a link.
(Other definitions for abiding that I've seen before include "Stopping" , "Tolerating" , "bearing" , "Established" , "firm" .)