Gather the animal on the inside is the dam (5)
I believe the answer is:
amass
'gather' is the definition.
(amass can mean to collect or gather)
'animal on the inside is the dam' is the wordplay.
'animal' becomes 'yak' (I have seen 'Tibetan animal ' mean 'yak' so perhaps 'animal' could also mean 'yak').
'on the inside' says to take the centre.
'is the dam' becomes 'mass' (I can't justify this - if you can you should give a lot more credence to this answer).
The middle letter of 'yak' is 'a'.
'a'+'mass'='AMASS'
'the' is the link.
Can you help me to learn more?
(Other definitions for amass that I've seen before include "Accumulate, pile up" , "Sodium chloride" , "Collect or gather" , "run up?" , "Collect, hoard" .)