Go by turns extremely expressionless and faint (6)
I believe the answer is:
elapse
'go by' is the definition.
(synonyms)
'turns extremely expressionless and faint' is the wordplay.
'turns' is a reversal indicator.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'and' means one lot of letters go next to another.
'faint' becomes 'pale' (I've seen this in another clue).
'expressionless' with its middle removed is 'es'.
'es'+'pale'='espale'
'espale' backwards is 'ELAPSE'.
(Other definitions for elapse that I've seen before include "Passing" , "Of time, pass by - please" , "run by" , "Please (anag) -- pass away" , "'Pass by, as time (6)'" .)