A second spasm, might one say? (4)
I believe the answer is:
tick
'a second' is the definition.
(both can mean a very short amount of time)
'spasm might one say?' is the wordplay.
'spasm' becomes 'tic' (tic is a kind of spasm).
'might one say?' shows a homophone (sound like).
'tic' is a homophone of 'TICK'.
(Other definitions for tick that I've seen before include "Clock's sound" , "credit > check" , "Make noise like old clock" , "Minute" , "Moment - parasitic arachnid" .)