Struggle with force during erection of bloody camera attachment! (10*)
I believe the answer is:
Here is my best explanation:
'camera attachment' is the definition.
'struggle with force during erection of bloody' is the subsidiary indication.
'struggle' becomes 'vie' (I've seen this in other clues).
'with force' becomes 'wf' (this might be a standard abbreviation of which I'm unaware).
'during' becomes 'in' (I've seen this in other clues).
'erection of' is a reversal indicator (in a down clue going up is a reversal).
'bloody' becomes 'red'.
'red' back-to-front is 'der'.
(Other definitions for viewfinder that I've seen before include "Sight", "part of camera", "Press to take picture after looking into it" and "camera attachment?".)