Volatile racer -- tricky to hold it back (7)
I believe the answer is:
erratic
'volatile' is the definition.
(I've seen this before)
'racer tricky to hold it back' is the wordplay.
'tricky' indicates an anagram.
'to hold' means one lot of letters goes inside another.
'back' is a reversal indicator.
'it' backwards is 'ti'.
'racer' anagrammed gives 'errac'.
'errac' going around 'ti' is 'ERRATIC'.
(Other definitions for erratic that I've seen before include "Liable to sudden unpredictable change" , "Haphazard" , "Inconsistent, shifting" , "Irregular or uneven in movement or habits" , "Irregular in motion or habit" .)