A squad lacks top assistant (4)
I believe the answer is:
aide
'assistant' is the definition.
(aide is a kind of assistant)
'a squad lacks top' is the wordplay.
'squad' becomes 'side'.
'lacks top' means to remove the first letter (I've seen 'lacking top' mean this).
'side' with its initial letter taken off is 'ide'.
'a'+'ide'='AIDE'
(Other definitions for aide that I've seen before include "Officer's assistant" , "Second" , "one helping" , "someone in assistance" , "Military officer acting as assistant" .)