Afternoon meal taken one by one isn't to be taken seriously (9)
I believe the answer is:
teasingly
'isn't to be taken seriously' is the definition.
I can't judge whether this definition defines the answer.
'afternoon meal taken one by one' is the wordplay.
'afternoon meal' becomes 'tea' (meal eaten in the afternoon).
'taken' says to put letters next to each other.
'one by one' becomes 'singly' (synonyms).
'tea'+'singly'='TEASINGLY'
Can you help me to learn more?
(Other definitions for teasingly that I've seen before include "not to be taken seriously!" , "in playful fashion" , "how some jokers behave" .)