Mathematically able to work out true mean (8)
I believe the answer is:
numerate
'mathematically able' is the definition.
(I've seen this before)
'work out true mean' is the wordplay.
'work out' is an anagram indicator.
'true'+'mean'='truemean'
'truemean' with letters rearranged gives 'NUMERATE'.
'to' is the link.
(Other definitions for numerate that I've seen before include "Able to do arithmetic" , "Capable of understanding mathematics" , "Good at maths and science" , "Mathematically able" , "Competent at arithmetic" .)