Acceptable rate of progress? I'm not so sure! (4)
I believe the answer is:
umph
'progress? i'm not so sure' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.
'acceptable rate' is the wordplay.
'acceptable' becomes 'u' (as in U and non-U speech).
'rate' becomes 'mph' (miles per hour).
'u'+'mph'='UMPH'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for umph that I've seen before include "sign of dissatisfaction" , "Expression of doubt" , "I doubt that! (interjec.)" , "showing dissatisfaction" .)