Vex in quite a serious way (5)
I believe the answer is:
tease
'vex' is the definition.
(I know this)
'in quite a serious way' is the wordplay.
'in' indicates a hidden word.
'TEASE' is hidden in the letters of 'quite a serious way'.
(Other definitions for tease that I've seen before include "Poke fun at" , "Kid" , "Rib, annoy" , "Banter, make gentle fun of" , "Comb to remove tangles and open out the fibres" .)