Having a better aim, say, finish getting into small opening (8)
I believe the answer is:
Here is my best explanation:
'having a better aim say' is the definition.
'finish getting into small opening' is the wordplay.
'finish' becomes 'die'.
'getting into' indicates putting letters inside.
'small' becomes 's' (abbreviation - e.g. clothes size).
'opening' becomes 'tear' (tear is a kind of opening**).
'die' put inside 'stear' is 'STEADIER'.
(Other definitions for steadier that I've seen before include "stabilising device", "Better balanced", "More firm or constant", "Less liable to fluctuate", "Readiest (anag.)", "More stable, being readiest", "More regular or constant", "More stable and reliable" and "More stabe, at readiest".)