It's viewed as what followed Labour's Callaghan era? (7)
I believe the answer is:
footage
'it's viewed' is the definition.
(film footage is viewed)
'followed labour's callaghan era?' is the wordplay.
'followed' is a charade indicator (letters next to each other).
'labour's callaghan' becomes 'foot' (I can't explain this - if you can you should believe this answer much more).
'era?' becomes 'age' (I've seen this before).
'foot' next to 'age' is 'FOOTAGE'.
'as what' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
(Other definitions for footage that I've seen before include "Piece of filmed material" , "Length of film" , "amount of film" , "Quantity of film (of event)" , "quantity shot" .)