Fastener joins deer and baby animal up (6)
I believe the answer is:
buckle
'fastener' is the definition.
(buckle is a kind of fastener)
'deer and baby animal up' is the wordplay.
'deer' becomes 'elk' (type of deer).
'and' means one lot of letters go next to another.
'baby animal' becomes 'cub' (cub is a kind of animal. I am not sure about the 'baby' bit.).
'up' is a reversal indicator (in down clue: letters go upwards).
'elk'+'cub'='elkcub'
'elkcub' backwards is 'BUCKLE'.
'joins' acts as a link.
Can you help me to learn more?
(Other definitions for buckle that I've seen before include "cave in under pressure?" , "Crumple under pressure, like shoe clasp" , "Catch" , "Fastener on belt or shoe" , "(E.g. belt) fastener; crumple" .)