Mark’s about to enlist (4,2)
I believe the answer is:
sign on
'enlist' is the definition.
(I've seen this before)
'mark's about' is the wordplay.
'mark' becomes 'sign' (synonyms).
'about' becomes 'on'.
'sign'+'on'='SIGN ON'
'to' is the link.
(Other definitions for sign on that I've seen before include "'Register, for job-seeker's allowance, say (4,2)'" , "claim unemployment benefit as well" , "Register oneself as for employment assistance" , "Register as for social welfare payments" , "Enlist" .)