Plant in enclosure confused tailless monster (9)
I believe the answer is:
penstemon
'plant' is the definition.
'enclosure confused tailless monster' is the wordplay.
'enclosure' becomes 'pen' (pen is a kind of enclosure).
'confused' indicates an anagram.
'tailless' means to remove the last letter.
'monster' with its last letter taken away is 'monste'.
'monste' anagrammed gives 'stemon'.
'pen'+'stemon'='PENSTEMON'
'in' is the link.
(Other definitions for penstemon that I've seen before include "showy flower" , "plant" , "Step on men, strangely, to get to the flower" .)