Band in grand merriment, going topless (5)
I believe the answer is:
girth
'band' is the definition.
(I've seen this before)
'grand merriment going topless' is the wordplay.
'grand' becomes 'g' (G is short for ground as in a thousand).
'merriment' becomes 'mirth' (mirth is a kind of merriment).
'going topless' means to remove the first letter.
'mirth' with its initial letter removed is 'irth'.
'g'+'irth'='GIRTH'
'in' acts as a link.
(Other definitions for girth that I've seen before include "Measure all round" , "Bulk" , "Body's circumference around the middle" , "Band around a horse's belly" , "Some tack" .)