Monsters making us go up about seconds (5)
I believe the answer is:
ogres
'monsters' is the definition.
(ogre is a type of monster)
'go up about seconds' is the wordplay.
'up' is a reversal indicator (in down clue: letters go upwards).
'about' becomes 're' ('re' means regarding or about).
'seconds' becomes 's'.
'go' back-to-front is 'og'.
'og'+'re'+'s'='OGRES'
'making us' acts as a link.
(Other definitions for ogres that I've seen before include "Giant types" , "intimidating people" , "Gorse (anag.)" , "Fairytale giants" , "She-giant of fairy tale" .)