Nothing to smile about — that's a bit below the belt (5)
I believe the answer is:
groin
'a bit below the belt' is the definition.
I can't judge whether this defines the answer.
'nothing to smile about' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'to smile' becomes 'grin' (grinning is a kind of smiling).
'about' is an insertion indicator.
'o' going inside 'grin' is 'GROIN'.
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for groin that I've seen before include "On rig (anag.)" , "Fold between belly and thigh" , "private parts" .)