Turn, possibly flat out, making an error (5)
I believe the answer is:
fault
'an error' is the definition.
(synonyms)
'turn possibly flat out' is the wordplay.
'turn' becomes 'u' (a 'U-turn' is a kind of turn).
'possibly' indicates an anagram.
'out' indicates putting letters inside.
'flat' with letters rearranged gives 'falt'.
'u' going into 'falt' is 'FAULT'.
'making' is the link.
(Other definitions for fault that I've seen before include "bad service" , "Defect; blunder" , "Censure" , "An incorrect serve in tennis" , "Defect; culpability" .)