Frogman tried at first to change course (6)
I believe the answer is:
divert
'change course' is the definition.
'frogman tried at first' is the wordplay.
'frogman' becomes 'diver' ('diver' can be a synonym of 'frogman').
'at first' says to take the initial letters.
The initial letter of 'tried' is 't'.
'diver'+'t'='DIVERT'
'to' acts as a link.
(Other definitions for divert that I've seen before include "Amuse or send another way" , "Entertain; put off course" , "Turn aside and amuse" , "Entertain; reroute" , "Amuse or send in another direction" .)