Once again look at ace in crimson garment (9)
I believe the answer is:
readdress
'once again look at' is the definition.
(to look at or address again)
'ace in crimson garment' is the wordplay.
'ace' becomes 'a' (abbreviation).
'in' means one lot of letters goes inside another.
'crimson' becomes 'red' (synonyms).
'garment' becomes 'dress' (dress is a type of garment).
'a' put within 'red' is 'read'.
'read'+'dress'='READDRESS'
(Other definitions for readdress that I've seen before include "tackle again" , "Have another look at" , "forward" , "change direction" .)