Island: old man therein crossing river, doing the backstroke? (8)
I believe the answer is:
trinidad
'island' is the definition.
(Trinidad is an example)
'old man therein crossing river doing the backstroke?' is the wordplay.
'old man' becomes 'dad' ('dad' can be a synonym of 'old man').
'therein' becomes 'init' (in it).
'crossing' means one lot of letters goes inside another.
'river' becomes 'r'.
'doing the backstroke?' says the letters should be written backwards.
'dad'+'init'='dadinit'
'dadinit' placed around 'r' is 'dadinirt'.
'dadinirt' backwards is 'TRINIDAD'.
(Other definitions for trinidad that I've seen before include "West Indian island paired with Tobago" , "Island linked to Tobago" , "English-speaking island off Venezuela" , "... [WI ISLAND]" , "Port of Spain island" .)