Seriously impressed when Brave Heart tied the knot (4)
I believe the answer is:
awed
'seriously impressed' is the definition.
(I've seen this before)
'brave heart tied the knot' is the wordplay.
'heart' means to look at the middle letters.
'tied the knot' becomes 'we'd' (I've seen this in another clue).
The centre of 'brave' is 'a'.
'a'+'wed'='AWED'
'when' is the link.
(Other definitions for awed that I've seen before include "Overcome with wonder or amazement" , "Made to feel fear/wonder" , "Filled with respect (or dread)" , "in terror" , "Filled with reverential wonder" .)