Retrospectively wish you could change uniform on guards? (3)
I believe the answer is:
rue
'retrospectively wish you could change' is the definition.
I can't judge whether this definition defines the answer.
'uniform on guards?' is the wordplay.
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'on' becomes 're' ('on' can be mean 'with reference to').
'guards?' indicates putting letters inside (inserted letters are guarded).
'u' put into 're' is 'RUE'.
Can you help me to learn more?
(Other definitions for rue that I've seen before include "Plant producing oil" , "Regret the bitter herb" , "Feel regret for the herb" , "show regret" , "Feel sorry" .)