Adults can be wrong on purpose? Nothing odd in that (5-3)

'adults' is the definition.
(grown-up is a kind of adult)

'can be wrong on purpose? nothing odd in that' is the wordplay.
'can be' indicates anagramming the letters.
'on' means one lot of letters go next to another.
'nothing odd in that' means one should take alternating letters (letters in odd-numbered positions are removed).
The alternating letters of 'purpose' are 'ups'.
'wrong' anagrammed gives 'grown'.
'grown'+'ups'='GROWN-UPS'