Giants making us go up about seconds (5)
I believe the answer is:
ogres
'giants' is the definition.
(I know this)
'go up about seconds' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'about' becomes 're' ('re' means regarding or about).
'seconds' becomes 's'.
'go' in reverse letter order is 'og'.
'og'+'re'+'s'='OGRES'
'making us' is the link.
(Other definitions for ogres that I've seen before include "'Monsters, fiends (5)'" , "Giant types" , "intimidating people" , "Beasts" , "She-giant of fairy tale" .)