Clue: Herb avoids middle one of four bars at Lord's (4)
Answer: BAIL
Explanation: Definition: one of four bars at Lord's
BA(s)IL ("herb", with *middle* letter *avoided*). 
*For those not au fait with cricket, the bail is the small piece of fashioned wood that sits on top of the wicket, two on each, thus "one of four bars at Lord's (cricket ground)"