Figures fixed, Soviet leader claims first (10)
I believe the answer is:
statistics
'figures' is the definition.
(statistical figures)
'fixed soviet leader claims first' is the wordplay.
'fixed' becomes 'static' (I've seen this before).
'leader' indicates taking the first letters.
'claims' is an insertion indicator.
'first' becomes 'ist' (ist resembles sst).
The initial letter of 'soviet' is 's'.
'static'+'s'='statics'
'statics' going around 'ist' is 'STATISTICS'.
(Other definitions for statistics that I've seen before include "'20, 16 20, and _____'" , "Branch of maths" , "results" , "(Study of) quantitative data" .)