Most meagre spring initially spilling into others (8)
I believe the answer is:
sparsest
'most meagre' is the definition.
(sparse can mean meagre or minimal)
'spring initially spilling into others' is the wordplay.
'spring' becomes 'spa' (spa can mean a mineral spring).
'initially' suggests taking the first letters.
'into' indicates putting letters inside.
'others' becomes 'rest' (eg the rest of a group).
The initial letter of 'spilling' is 's'.
's' inserted within 'rest' is 'rsest'.
'spa'+'rsest'='SPARSEST'
(Another definition for sparsest that I've seen is " Most scattered".)