Constant in aim, ruler strangely unpredictable (9)
I believe the answer is:
mercurial
'unpredictable' is the definition.
(synonyms)
'constant in aim ruler strangely' is the wordplay.
'constant' becomes 'C' (mathematical abbreviation).
'in' indicates putting letters inside.
'strangely' indicates anagramming the letters.
'aim'+'ruler'='aimruler'
'aimruler' is an anagram of 'merurial'.
'c' inserted within 'merurial' is 'MERCURIAL'.
(Other definitions for mercurial that I've seen before include "lively" , "rapidly changing" , "Temperamental" , "Changeable in mood" , "Changing unpredictably" .)