Most squat when returning mud pies thrown first (8)

'most squat' is the definition.
('dumpy' can be a synonym of 'squat')

'returning mud pies thrown first' is the wordplay.
'returning' says the letters should be written backwards.
'first' says to take the initial letters.
The initial letter of 'thrown' is 't'.
'mud' written backwards gives 'dum'.
'dum'+'pies'+'t'='DUMPIEST'

'when' is the link.