Trying to get a foot on the bottom of the ladder (5)
I believe the answer is:
after
'trying to get' is the definition.
(someone 'after' something is trying to get it)
'a foot on the bottom of the ladder' is the wordplay.
'foot' becomes 'ft' (abbreviation).
'on' is a charade indicator (letters next to each other).
'the bottom of' indicates one should take the final letters.
The final letters of 'the ladder' are 'er'.
'a'+'ft'+'er'='AFTER'
(Other definitions for after that I've seen before include "In the time following" , "Behind; in quest of" , "Looking for" , "In honour of" , "At a later time or behind" .)