Stranger that is caught by on-line blunder (6)
I believe the answer is:
eerier
'stranger' is the definition.
(I've seen this before)
'that is caught by on-line blunder' is the wordplay.
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'caught by' means one lot of letters goes inside another.
'on-line' becomes 'e' (abbreviation for electronic, e.g. in ecommerce).
'blunder' becomes 'err' (I've seen this before).
'e'+'err'='eerr'
'ie' inserted into 'eerr' is 'EERIER'.
(Other definitions for eerier that I've seen before include "More unsettling" , "More mysterious" , "More strange and frightening" , "Spookier" , "More weird" .)