Senior officer in company with a learner (3)
I believe the answer is:
col
'senior officer' is the definition.
(abbreviation for colonel)
'company with a learner' is the wordplay.
'company' becomes 'co' (abbreviation).
'with' says to put letters next to each other.
'a learner' becomes 'l' (as in a learner driver with L-plates on their car).
'co'+'l'='COL'
'in' acts as a link.
(Other definitions for col that I've seen before include "High mountain pass" , "Low point of a ridge" , "Officer" , "Young horse almost" , "Somewhere up on the mountain" .)