Seaman oddly seeing crew showing non-attendance (7)
I believe the answer is:
absence
'non-attendance' is the definition.
(I've seen this before)
'seaman oddly seeing crew' is the wordplay.
'seaman' becomes 'ab'.
'oddly' indicates alternate letters.
'seeing'+'crew'='seeingcrew'
The alternate letters of 'seeingcrew' are 'sence'.
'ab'+'sence'='ABSENCE'
'showing' acts as a link.
(Other definitions for absence that I've seen before include "What supposedly makes the heart grow fonder?" , "It makes the heart grow fonder, they say" , "Failure to show" , "State of being away" , "reason for better relationship?" .)