Mark is amongst sailors retreated for worms (9)

'worms' is the definition.
(nematode is a kind of worm)

'mark is amongst sailors retreated' is the wordplay.
'mark' becomes 'dot' (dotting is a kind of marking).
'is amongst' indicates putting letters inside (I've seen this in other clues).
'sailors' becomes 'seamen' (seaman is a kind of sailor**).
'retreated' is a reversal indicator (I've seen 'retreat' mean this).
'dot' placed into 'seamen' is 'sedotamen'.
'sedotamen' in reverse letter order is 'NEMATODES'.

'for' is the link.