Front of bow, say, in launch (5)
I believe the answer is:
begin
'launch' is the definition.
(thesaurus)
'front of bow say in' is the wordplay.
'front of' indicates taking the first letters.
'say' becomes 'eg'.
The initial letter of 'bow' is 'b'.
'b'+'eg'+'in'='BEGIN'
(Other definitions for begin that I've seen before include "Partly open" , "Get cracking" , "Initiate" , "Israeli prime minister and Nobel Peace Prize winner, d. 1992" , "Israeli statesman" .)