... and uncertain expression is to lose freshness (6)
I believe the answer is:
wither
'to lose freshness' is the definition.
(I've seen this in another clue)
'and uncertain expression' is the wordplay.
'and' becomes 'with'.
'uncertain expression' becomes 'er' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'with'+'er'='WITHER'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for wither that I've seen before include "Writhe about and shrivel" , "Dry up, decay" , "Dry up, shrivel (6)" , "Shrink, shrivel up" , "(Of a plant) shrivel up" .)