Deep regret about Endeavour? (7)
I believe the answer is:
remorse
'deep regret' is the definition.
(I've seen this before)
'about endeavour?' is the wordplay.
'about' becomes 're' (common abbreviation for 'with reference to').
'endeavour?' becomes 'morse' (I've seen this before).
're'+'morse'='REMORSE'
(Other definitions for remorse that I've seen before include "display of conscience" , "compassion" , "Sorrow for wrong committed" , "Contrition for wrongdoing" , "Feeling of regret for wrongdoing" .)