Running flat out, and initially excessively (2,1,5)
I believe the answer is:
to a fault
'excessively' is the definition.
('to a fault' can be a synonym of 'excessively')
'running flat out and initially' is the wordplay.
'running' indicates anagramming the letters.
'initially' suggests taking the first letters.
The initial letter of 'and' is 'a'.
'flat'+'out'+'a'='flatouta'
'flatouta' is an anagram of 'TO A FAULT'.
(Other definitions for to a fault that I've seen before include "Where bad service leads" , "bad driver finally disqualified, taking blame" , "Where sparks might be drawn" , "Excessively, like being too generous" , "Excessively, like over-generosity" .)