Cups in large boxes right? That's a mental lapse (5,4)

'a mental lapse' is the definition.

'cups in large boxes right?' is the wordplay.
'cups' becomes 'bra' (as in the two cups of a bra).
'large' becomes 'fat' (I've seen this in other clues).
'boxes' indicates putting letters inside (the outside letters must 'box' inside letters in).
'right?' becomes 'R' (common abbreviation).
'fat' enclosing 'r' is 'fart'.
'bra'+'in'+'fart'='BRAIN FART'

'that's' acts as a link.