The speed extraterrestrials have (4)
I believe the answer is:
rate
'the speed' is the definition.
(both can mean pace)
'extraterrestrials have' is the wordplay.
'have' indicates a hidden word.
'RATE' can be found hidden inside 'extraterrestrials'.
(Other definitions for rate that I've seen before include "Cost" , "Adjudge" , "Tear up in tempo" , "Estimate value of - speed" , "Rank; speed" .)