Aboard, crew member is available (2,4)
I believe the answer is:
on hand
'available' is the definition.
(similar in meaning)
'aboard crew member' is the wordplay.
'aboard' becomes 'on' (eg on a ship or train).
'crew member' becomes 'hand' (I've seen this in another clue).
'on'+'hand'='ON HAND'
'is' is the link.
(Other definitions for on hand that I've seen before include "Present or easily available" , "Where digital process is" , "Available like wedding ring?" , "Wedding ring is normally" , "location of bananas?" .)