Sick donkey or toy bear may be incompatible (3-8)
I believe the answer is:
ill-assorted
'incompatible' is the definition.
'sick donkey or toy bear' is the wordplay.
'sick' becomes 'ill' (synonyms).
'donkey' becomes 'ass' (ass is a term for a donkey).
'toy bear' becomes 'ted' (short for teddy bear).
'ill'+'ass'+'or'+'ted'='ILL-ASSORTED'
'may be' acts as a link.
(Other definitions for ill-assorted that I've seen before include "Not matching" , "Unsuitably matched" .)