Pack short underwear that's wild (7)
I believe the answer is:
rampant
'wild' is the definition.
(I've seen this before)
'pack short underwear' is the wordplay.
'pack' becomes 'ram' ('ram' can be a synonym of 'pack').
'short' means to remove the last letter.
'underwear' becomes 'pants' (I've seen this before).
'pants' with its final letter taken off is 'pant'.
'ram'+'pant'='RAMPANT'
'that's' is the link.
(Other definitions for rampant that I've seen before include "On the rampage" , "Rife" , "Mantrap (anag) - unrestrained" , "Spreading unchecked" , "Unrestrained and violent" .)