Reach Ottawa's centre in a vessel (6)
I believe the answer is:
attain
'reach' is the definition.
(I've seen this before)
'ottawa's centre in a vessel' is the wordplay.
'centre' means to look at the middle letters.
'in' means one lot of letters goes inside another.
'vessel' becomes 'tin' (I've seen this in other clues**).
The central letters of 'ottawa' are 'ta'.
'a'+'tin'='atin'
'ta' going within 'atin' is 'ATTAIN'.
(Other definitions for attain that I've seen before include "Arrive at, accomplish" , "Gain with effort" , "Reach with effort" , "Succeed in reaching, achieve" , "Achieve with effort" .)