Former marine ultimately boards punctually (3-4)
I believe the answer is:
one-time
'former' is the definition.
(I've seen this before)
'marine ultimately boards punctually' is the wordplay.
'ultimately' says to take the final letters.
'boards' indicates putting letters inside.
'punctually' becomes 'on time' (I've seen this in another clue).
The final letter of 'marine' is 'e'.
'e' placed into 'ontime' is 'ONE-TIME'.
(Other definitions for one-time that I've seen before include "Relating to a single occasion" , "Former" .)