Impressive when attacking (8)
I believe the answer is:
striking
I believe this is a double definition.
'impressive' is the first definition.
('striking' can be similar in meaning to 'impressive')
'attacking' is the second definition.
(striking is a kind of attacking)
'when' is the link.
(Other definitions for striking that I've seen before include "Conspicuous" , "going like the clappers?" , "Conspicuously impressive" , "Attracting attention by reason of being unusual" , "Impressive; refusing to work" .)