Sailor heads off upwards, securing a lanyard's end (3,3)
I believe the answer is:
sea dog
'sailor' is the definition.
(I know that sea dog is a type of sailor)
'heads off upwards securing a lanyard's end' is the wordplay.
'heads off' becomes 'goes' (I've seen this in another clue).
'upwards' is a reversal indicator.
'securing' indicates putting letters inside.
'end' suggests the final letters.
The final letter of 'lanyard' is 'd'.
'goes' written backwards gives 'seog'.
'a'+'d'='ad'
'seog' placed around 'ad' is 'SEA-DOG'.
(Other definitions for sea dog that I've seen before include "mythical creature" , "Experienced sailor (colloq.)" , "'Old salt (3,3)'" , "Experienced mariner" , "An old sailor of canine origin" .)