Progress slowly with polar explorer almost formless (8)
I believe the answer is:
inchoate
'formless' is the definition.
(inchoate can mean lacking form)
'progress slowly with polar explorer almost' is the wordplay.
'progress slowly' becomes 'inch' (to inch is to progress forwards slowly).
'with' says to put letters next to each other.
'polar explorer' becomes 'Oates' (Titus Oates).
'almost' means to remove the last letter (most of the word but not all of it).
'oates' with its last letter taken off is 'oate'.
'inch'+'oate'='INCHOATE'
(Other definitions for inchoate that I've seen before include "Rudimentary, not fully formed" , "Not established" , "Confused" , "Only half-formed" , "Rough" .)