Stranger's hesitant expression that is caught by another (6)
I believe the answer is:
eerier
'stranger's' is the definition.
(I've seen this before)
'hesitant expression that is caught by another' is the wordplay.
'hesitant expression' becomes 'er'.
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'caught by' is an insertion indicator.
'another' becomes 'er' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'er'+'ie'='erie'
'erie' put inside 'er' is 'EERIER'.
(Other definitions for eerier that I've seen before include "More strangely frightening" , "More uncanny" , "Stranger" , "More weird" , "More strange and frightening" .)