Reach volunteers aboard a vessel (6)
I believe the answer is:
attain
'reach' is the definition.
(I've seen this before)
'volunteers aboard a vessel' is the wordplay.
'volunteers' becomes 'ta' (abbreviation for Territorial Army).
'aboard' means one lot of letters goes inside another.
'vessel' becomes 'tin' (I've seen this in other clues**).
'a'+'tin'='atin'
'ta' put within 'atin' is 'ATTAIN'.
(Other definitions for attain that I've seen before include "Succeed in getting or doing" , "Bring off" , "Reach or gain" , "Arrive at, accomplish" , "Reach (a specified level)" .)