Way of working on part of test in addition (8)
I believe the answer is:
moreover
'in addition' is the definition.
(synonyms)
'way of working on part of test' is the wordplay.
'way of working' becomes 'mo' (abbreviation for modus operandi).
'on' becomes 're' ('on' can be mean 'with reference to').
'part of test' becomes 'over' (an over might be part of test cricket).
'mo'+'re'+'over'='MOREOVER'
(Other definitions for moreover that I've seen before include "Furthermore, in addition" , "Besides, in addition" , "as well" .)