A bet involving millions? It makes you shudder (7)
I believe the answer is:
tremble
'shudder' is the definition.
(I know this)
'a bet involving millions?' is the wordplay.
'a bet' becomes 'treble' (treble is a type of bet).
'involving' indicates putting letters inside.
'millions?' becomes 'm' (abbreviation).
'treble' placed around 'm' is 'TREMBLE'.
'it makes you' is the link.
(Other definitions for tremble that I've seen before include "Shiver, quake" , "Shake with fright or cold" , "Quakers do this" , "get the shakes" , "Shake with fear or excitement" .)