Coach following Everton's lead certainly causes tension for viewers (9)
I believe the answer is:
Here is my best explanation:
'tension for viewers' is the definition.
'coach following everton's lead certainly' is the subsidiary indication.
'coach' becomes 'train' ('train' can be a synonym of 'coach'**).
'following' means one lot of letters go next to another.
'lead' says to take the initial letters.
'certainly' becomes 'yes' (I've seen this before).
The first letter of 'everton' is 'e'.
'train' after 'eyes' is 'eyestrain'.
'causes' is the link.
(Other definitions for eyestrain that I've seen before include "Vision tiredness", "Problem for pupils", "Eat resiny mix to cause optical pressure", "discomfort", "It might cause pupils problem", "Optical irritation or tiredness", "Tension on the visual organ", "Difficulty in sight", "Difficulty for reader" and "Tiredness when reading".)