Petty playing about involving third of set (8)
I believe the answer is:
footling
'petty' is the definition.
(I know that petty can be written as footling)
'playing about involving third of set' is the wordplay.
'playing about' becomes 'fooling' (fooling around).
'involving' means one lot of letters goes inside another.
'third of set' becomes 't' (3rd letter of 'set').
'fooling' enclosing 't' is 'FOOTLING'.
(Other definitions for footling that I've seen before include "foolish" , "Trivial, petty" , "pretty trivial" , "Trivial, irritating" , "of little consequence" .)