About to attach unknown container (5)
I believe the answer is:
caddy
'container' is the definition.
(caddy is a kind of container)
'about to attach unknown' is the wordplay.
'about' becomes 'c' (abbreviation for circa).
'to attach' becomes 'add' (I've seen this in another clue).
'unknown' becomes 'y' (algebra).
'c'+'add'+'y'='CADDY'
(Other definitions for caddy that I've seen before include "Tea box" , "Assistant for one wanting a good round" , "course attendant?" , "In which Earl Grey may be" , "supplier of tea" .)