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 definition 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' indicates putting letters inside.
'o' going within 'grin' is 'GROIN'.
'that's' is the 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" .)