Oddly sick sailor getting blackleg (4)
I believe the answer is:
scab
'blackleg' is the definition.
(I know that blackleg can be written as scab)
'oddly sick sailor getting' is the wordplay.
'oddly' means one should take alternating letters (letters in odd-numbered positions).
'sailor' becomes 'ab' (abbreviation for able seaman).
'getting' says to put letters next to each other.
The alternate letters of 'sick' are 'sc'.
'sc'+'ab'='SCAB'
(Other definitions for scab that I've seen before include "Evidence of healing wound - crosser of picket line" , "work when others are out" , "cause of itch?" , "Crust that forms on a healing skin wound" , "Ignorer of strike" .)