Number in rowing team on Severn occasionally (8)
I believe the answer is:
eighteen
'number' is the definition.
(I know that eighteen is a heptagonal number)
'rowing team on severn occasionally' is the wordplay.
'rowing team' becomes 'eight' (eight rowers).
'on' says to put letters next to each other.
'occasionally' indicates alternate letters.
The alternating letters of 'severn' are 'een'.
'eight'+'een'='EIGHTEEN'
'in' acts as a link.
(Other definitions for eighteen that I've seen before include "Age of electoral majority in UK" , "when boy becomes man?" , "Number of on-field players in an Australian Rules football team" , "age of maturity" , "Nine times two" .)