What old sailors must follow to go beyond headland (4,4)
I believe the answer is:
cape horn
'headland' is the definition.
'what old sailors must follow to go beyond' is the wordplay.
'what' becomes 'eh' (I've seen this in other clues).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'sailors' becomes 'rn' (abbreviation for Royal Navy).
'must follow' means one lot of letters go next to another.
'to go beyond' becomes 'cap'.
'eh'+'o'+'rn'='ehorn'
'ehorn' after 'cap' is 'CAPE HORN'.
(Other definitions for cape horn that I've seen before include "headland - with 2d" , "Southernmost point of Chile" , "Chaperon in most southerly part of S America" , "Most southerly point of South America" , "Notorious Chilean headland" .)