Shackleton's heading to visit pole or bust! (5)
I believe the answer is:
bosom
'bust' is the definition.
(I've seen this in another clue)
'shackleton's heading to visit pole' is the wordplay.
'heading to' says to take the initial letters.
'visit' is an insertion indicator.
'pole' becomes 'boom' (boom is a kind of pole).
The first letter of 'shackleton' is 's'.
's' placed into 'boom' is 'BOSOM'.
'or' is the link.
(Other definitions for bosom that I've seen before include "Breast, chest" , "Woman's breast" , "A close affectionate and protective acceptance" , "Intimate" , "Woman's chest" .)