As seen by the couple about to back us (6)
I believe the answer is:
visual
'as seen' is the definition.
The answer and definition are not the same part of speech. However, adjectives and past participle verbs can sometimes define each other.
'the couple about to back us' is the wordplay.
'the couple' becomes 'vial' (I can't justify this - if you can you should believe this answer much more).
'about' is an insertion indicator.
'to back' shows that the letters should be reversed in order.
'us' backwards is 'su'.
'vial' going around 'su' is 'VISUAL'.
'by' is the link.
Can you help me to learn more?
(Other definitions for visual that I've seen before include "Of sight and seeing" , "Of the eyes or sight" , "Relating to, or using, sight" , "Relating to seeing" , "Pertaining to sight" .)