Starts to regret opening up garage - extremely cosmetic (5)
I believe the answer is:
rouge
'cosmetic' is the definition.
(I know that rouge is a type of make-up)
'starts to regret opening up garage extremely' is the wordplay.
'starts to' says to take the initial letters.
'extremely' means to remove the middle letters (only the extremes of the word are used).
'garage' with its middle taken out is 'ge'.
The first letters of 'regret opening up' is 'rou'.
'rou'+'ge'='ROUGE'
(Other definitions for rouge that I've seen before include "Make-up item" , "struggle at Eton" , "Reddish cosmetic for colouring cheeks" , "Blusher" , "Cheek colouring" .)