A heartless wife upset me — so formidable! (7)
I believe the answer is:
awesome
'formidable' is the definition.
(synonyms)
'a heartless wife upset me so' is the wordplay.
'heartless' suggests removing the centre.
'upset' is an anagram indicator.
'wife' with its middle removed is 'we'.
'me'+'so'='meso'
'meso' with letters rearranged gives 'some'.
'a'+'we'+'some'='AWESOME'
(Other definitions for awesome that I've seen before include "Very imposing, inspiring wonder" , "Exceptionally good" , "Hugely impressive, creating fear and wonder" , "Portentous" , "Really excellent" .)