There is nothing to some extent possible and doubtful in a marsupial (7)
I believe the answer is:
opossum
'marsupial' is the definition.
(I know that opossum is a type of marsupial)
'there is nothing to some extent possible and doubtful' is the wordplay.
I cannot really see how this works, but
'nothing' could be 'o' (looks like zero - 0) and 'o' is found in the answer.
'doubtful' could be 'um' (saying 'um' communicates doubt) and 'um' is located in the answer.
The remaining letters 'poss' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'in a' is the link.
Can you help me to learn more?
(Other definitions for opossum that I've seen before include "Small Australian marsupial" , "Thickly furred marsupial" , "Native of America" , "Small arboreal marsupial" , "American arboreal marsupial" .)