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 different parts of speech. However, adjectives and past participle verbs occasionally define each other.
'couple about to back us' is the wordplay.
'couple' becomes 'vial' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'about' means one lot of letters goes inside another.
'to back' says the letters should be written in reverse.
'us' reversed gives 'su'.
'vial' going around 'su' is 'VISUAL'.
'by the' 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" .)