Measure rodent against porcupine's tail (4)
I believe the answer is:
rate
'measure' is the definition.
(I've seen this before)
'rodent against porcupine's tail' is the wordplay.
'rodent' becomes 'rat' (rat is a kind of rodent).
'against' says to put letters next to each other.
'tail' indicates one should take the final letters.
The final letter of 'porcupine' is 'e'.
'rat'+'e'='RATE'
(Other definitions for rate that I've seen before include "Think highly of - price" , "Amount measured against another" , "Charge per unit" , "Value - scold" , "Adjudge" .)