Concerned with time machine forgetting one decelerates (7)
I believe the answer is:
retards
'decelerates' is the definition.
(retarding is a kind of decelerating)
'concerned with time machine forgetting one' is the wordplay.
'concerned with' becomes 're'.
'time machine' becomes 'tardis' (I've seen this before).
'forgetting' suggests deleting specific letters.
'one' becomes 'i' (Roman numeral).
'tardis' with 'i' taken out is 'tards'.
're'+'tards'='RETARDS'
(Other definitions for retards that I've seen before include "decelerates" , "Slows the growth of" , "Delays, slows the progress of" , "Holds back, slows down." .)