Seaman's sources of unusually strong bones (6)
I believe the answer is:
tarsus
'bones' is the definition.
(I've seen this before)
'seaman's sources of unusually strong' is the wordplay.
'seaman's' becomes 'tars' ('tar' can be a synonym of 'seaman'**).
'sources of' suggests taking the first letters.
The first letters of 'unusually strong' is 'us'.
'tars'+'us'='TARSUS'
(Other definitions for tarsus that I've seen before include "Birthplace of St Paul" , "Bones of the ankle and heel" , "The seven bones of the upper foot and ankle" , "City of which St Paul was a native" , "St Paul was born there" .)