Get going and, using every second, firm up belly (5)
I believe the answer is:
impel
'get going' is the definition.
(I've seen this in another clue)
'every second firm up belly' is the wordplay.
'every second' means one should take alternating letters (I've seen 'every second of' mean this).
'firm'+'up'+'belly'='firmupbelly'
The alternating letters of 'firmupbelly' are 'IMPEL'.
'and using' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for impel that I've seen before include "Motivate" , "Hold together" , "Force forwards" , "Press on - drive forward" , "Goad" .)