Performing their number in it, do really well (6)
I believe the answer is:
thrive
'do really well' is the definition.
I can't tell whether this definition defines the answer.
'performing their number in it' is the wordplay.
'performing' indicates anagramming the letters (the letters move around or 'perform').
'number' becomes 'v' (V is an example).
'in it' is an insertion indicator.
'their' anagrammed gives 'thrie'.
'thrie' enclosing 'v' is 'THRIVE'.
Can you help me to learn more?
(Other definitions for thrive that I've seen before include "Grow vigorously, flourish" , "Do well, flourish" , "Prosper, grow well" , "Prosper, flourish" , "Develop well" .)