Turn, possibly flat out, making an error (5)
I believe the answer is:
fault
'an error' is the definition.
('fault' can be a synonym of 'error')
'turn possibly flat out' is the wordplay.
'turn' becomes 'u' (a 'U-turn' is a kind of turn).
'possibly' indicates anagramming the letters.
'out' is an insertion indicator.
'flat' anagrammed gives 'falt'.
'u' inserted within 'falt' is 'FAULT'.
'making' acts as a link.
(Other definitions for fault that I've seen before include "bad service" , "Defect; blunder" , "Censure" , "An incorrect serve in tennis" , "Defect; culpability" .)