Back up leader of squad in former beat (7)
I believe the answer is:
pulsate
'beat' is the definition.
(pulsating is a kind of beating)
'back up leader of squad in former' is the wordplay.
'back' says the letters should be written in reverse.
'leader of' says to take the initial letters.
'in' is an insertion indicator.
'former' becomes 'late' (**).
The first letter of 'squad' is 's'.
'up' in reverse letter order is 'pu'.
's' inserted into 'late' is 'lsate'.
'pu'+'lsate'='PULSATE'
(Other definitions for pulsate that I've seen before include "Expand and contract regularly" , "thrill" , "Throb regularly like heart" , "Produce a regular throbbing sensation" .)