Extremely tough mug for pound (5)
I believe the answer is:
throb
'for pound' is the definition.
(throbbing is a kind of pounding)
'extremely tough mug' is the wordplay.
'extremely' means to remove the middle letters (only the extremes of the word are used).
'mug' becomes 'rob' (I've seen this in another clue).
'tough' with its centre removed is 'th'.
'th'+'rob'='THROB'
(Other definitions for throb that I've seen before include "Pulsate; regular beat" , "Pulsate with force" , "Have strong or irregular heartbeat" , "Pulsate strongly" , "beat repeatedly" .)