He can't be trusted in putting part of the fence up (4)
I believe the answer is:
liar
'he can't be trusted' is the definition.
(I've seen this in another clue)
'fence up' is the wordplay.
'fence' becomes 'rail fence' (rail fence is a kind of fence).
'up' is a reversal indicator (in down clue: letters go upwards).
'rail' reversed gives 'LIAR'.
'in putting part of the' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for liar that I've seen before include "Teller of untruths" , "one trying to sell pork pies" , "man of fiction?" , "Pinocchio, at times" , "One whose pants are on fire?" .)