Son far from straight in making complaint in the main? (6)
I believe the answer is:
scurvy
'complaint in the main?' is the definition.
(illness at sea - 'the main' can mean the sea)
'son far from straight' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'far from straight' becomes 'curvy' (having curves).
's'+'curvy'='SCURVY'
'in making' is the link.
(Other definitions for scurvy that I've seen before include "Sailors' old disease" , "problem for many a sailor?" , "Result of vitamin C deficiency" , "Condition caused by a deficiency of vitamin C" , "Disease caused by vitamin C deficiency" .)