One getting time if he messed up? (5)
I believe the answer is:
thief
'one getting' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'time if he messed up?' is the wordplay.
'time' becomes 't' (abbreviation).
'messed up?' indicates anagramming the letters (I've seen 'messed up' mean this).
'if'+'he'='ifhe'
'ifhe' is an anagram of 'hief'.
't'+'hief'='THIEF'
(Other definitions for thief that I've seen before include "Crook" , "Afternoon break" , "A robber or pickpocket" , "Aus. or Scots city" , "Burglar, say" .)