Unearth grub, oddly, during slump (3,2)
I believe the answer is:
dig up
'unearth' is the definition.
(thesaurus)
'grub oddly during slump' is the wordplay.
'oddly' indicates alternate letters.
'during' indicates putting letters inside.
'slump' becomes 'dip' (I've seen this before).
The alternating letters of 'grub' are 'gu'.
'gu' put into 'dip' is 'DIG UP'.
(Other definitions for dig up that I've seen before include "Disinter" , "Exhume" , "Excavate - discover" , "Uncover (something hidden)" , "Unearth" .)