"The exercise of restraint may well be difficult." I can understand the sentence if drop "well". does that mean the exercise of restraint may be difficult as well?(It is a sentence from a dictionary, so I don't have the context."
'Well' here acts as an intensifier to 'may'. If we say something may happen or be true, we are just saying that it could happen or be true. If we say it may well happen or be true, we are saying there is a strong chance that it will happen, or that it probably will happen.