Boycott bar for not letting the workers in
I believe the answer is:
picket line
'not letting the workers in' is the definition.
I can't tell whether this defines the answer.
'boycott bar' is the wordplay.
'boycott' becomes 'picket' (I can't explain this - if you can you should give a lot more credence to this answer).
'bar' becomes 'line' (I've seen this in other clues).
'picket'+'line'='PICKET LINE'
'for' is the link.
Can you help me to learn more?
(Other definitions for picket line that I've seen before include "Some in labour dispute" , "Striking protest" , "Strike enforcers" , "Barrier established by strikers" .)