Sailor taking on boundless new career (4)
I believe the answer is:
tear
'career' is the definition.
(I've seen this in another clue)
'sailor taking on boundless new' is the wordplay.
'sailor' becomes 'tar' (term for a sailor).
'taking on' indicates putting letters inside.
'boundless' means to look at the middle letters (the limits or bounds of the word are removed).
The centre of 'new' is 'e'.
'tar' going around 'e' is 'TEAR'.
(Other definitions for tear that I've seen before include "wrench" , "charge" , "Come apart" , "Drop secreted by the lacrimal glands" , "Rate (anag.)" .)