Reptiles, concealed in topiary, go outside no longer (7)
I believe the answer is:
ophidia
'reptiles' is the definition.
(ophidia is a group of reptiles)
'concealed in topiary go outside no longer' is the wordplay.
I cannot really understand how this works, but
'concealed' could be 'hid' ('hide' can be a synonym of 'conceal') and 'hid' is present in the answer.
'no' could be 'o' (O resembles 0) and 'o' is found in the answer.
The remaining letters 'pia' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
(Another definition for ophidia that I've seen is " [SNAKES]".)