Marks and Spencer's opening times assuming it's glossy magazine's accurate for starters - ace (8)
I believe the answer is:
stigmata
'marks' is the definition.
(plural of stigma - stigma can mean a mark or wound)
'assuming it's glossy magazine's accurate for starters ace' is the wordplay.
'assuming' says to put letters next to each other.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'glossy' becomes 'stig' (I am not sure about this - if you are sure you should believe this answer much more).
'for starters' indicates taking the first letters.
'ace' becomes 'a' (abbreviation).
The initial letters of 'magazines accurate' is 'ma'.
'stig'+'ma'='stigma'
't' put after 'stigma' is 'stigmat'.
'stigmat'+'a'='STIGMATA'
'and spencer's opening times' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for stigmata that I've seen before include "Spots" , "Disgraces" , "Part of flower" , "Marks resembling the wounds of Christ" , "Distinguishing marks; the wounds of Christ" .)