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 "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".)