Fragrant vapour having appeared intermittently outside (8)
I believe the answer is:
perfumed
'fragrant' is the definition.
(similar in meaning)
'having appeared intermittently outside' is the wordplay.
'having' becomes 'fume' (I am not sure about this - if you are sure you should believe this answer much more).
'intermittently' means one should take alternating letters.
'outside' indicates putting letters inside (some letters go outside others).
The alternating letters of 'appeared' are 'perd'.
'fume' put into 'perd' is 'PERFUMED'.
'vapour' is the link.
This may not be correct. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for perfumed that I've seen before include "Scented pp" , "tempting aroma" , "Fragrant" , "With a natural fragrance" .)