Rower's first to enter large vessel (4)
I believe the answer is:
brig
'vessel' is the definition.
(brig is a kind of vessel)
'rower's first to enter large' is the wordplay.
'first' says to take the initial letters.
'to enter' is an insertion indicator.
'large' becomes 'big' ('big' can be a synonym of 'large'**).
The first letter of 'rowers' is 'r'.
'r' going within 'big' is 'BRIG'.
(Other definitions for brig that I've seen before include "Sailing vessel or lock-up on board" , "Ship; ship's prison" , "'Two-masted,square-rigged ship (4)'" , "staff officer briefly used" , "senior officer briefly used" .)