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 subsidiary indication.
'finish' becomes 'die'.
'getting into' means one lot of letters goes inside another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'opening' becomes 'tear' (tear is a kind of opening**).
'die' going within 'stear' is 'steadier'.
(Other definitions for steadier that I've seen before include "More stable and reliable", "Less liable to fluctuate", "Readiest (anag.)", "More stabe, at readiest", "More regular or constant", "More stable, being readiest" and "More firm or constant".)