Opening mimic about to admit loveless tour (8)
I believe the answer is:
aperture
'opening' is the definition.
(aperture is a kind of opening)
'mimic about to admit loveless tour' is the wordplay.
'mimic' becomes 'ape' (I've seen this before).
'about to' becomes 're' (I've seen this in other clues).
'admit' means one lot of letters goes inside another.
'loveless tour' becomes 'tur' (I can't explain this - if you can you should believe this answer much more).
're' going around 'tur' is 'rture'.
'ape'+'rture'='APERTURE'
Can you help me to learn more?
(Other definitions for aperture that I've seen before include "Opening (on camera, for example)" , "Opening, to let light enter a camera perhaps" , "Eye" , "Crack" , "Opening, mostly small" .)