Bull and mare desperate for shelter (8)
I believe the answer is:
umbrella
'shelter' is the definition.
(umbrella is a kind of shelter)
'bull and mare desperate' is the wordplay.
'and' says to put letters next to each other.
'desperate' is an anagram indicator.
'bull'+'mare'='bullmare'
'bullmare' with letters rearranged gives 'UMBRELLA'.
'for' is the link.
(Other definitions for umbrella that I've seen before include "protective cover" , "Parasol" , "Protection that can be rolled up" , "Collapsible ribbed canopy against rain" , "one should keep water away" .)