Erratic shooting ultimately costly (5)
I believe the answer is:
flaky
'erratic' is the definition.
(thesaurus)
'shooting ultimately costly' is the wordplay.
'shooting' becomes 'flak' (I've seen this before).
'ultimately' indicates one should take the final letters.
The last letter of 'costly' is 'y'.
'flak'+'y'='FLAKY'
(Other definitions for flaky that I've seen before include "Easily separating; unreliable" , "Tending to break or peel off - crazy" , "pastry" .)