Conversation starter that bloke will duck (5)
I believe the answer is:
hello
'conversation starter' is the definition.
(I've seen this before)
'bloke will duck' is the wordplay.
'bloke will' becomes 'hell' ('he'll', contraction of 'he will').
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'hell'+'o'='HELLO'
'that' acts as a link.
(Other definitions for hello that I've seen before include "Common greeting as on phone" , "royals dominate its pages?" , "Routine response to telephone call" , "Conversation starter" , "Expression of greeting" .)