Gather the animal on the inside is the dam (5)
I believe the answer is:
amass
'gather the animal' is the definition.
'amass' can be an answer for 'gather' ('amass' can be a synonym of 'gather'). I am unsure of the 'the animal' bit.
'on the inside is the dam' is the wordplay.
'on' becomes 'ready' ('ready' can be a synonym of 'on').
'the inside' means to look at the middle letters.
'is the dam' becomes 'mass' (I can't explain this - if you can you should give a lot more credence to this answer).
The middle of 'ready' is 'a'.
'a'+'mass'='AMASS'
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" .)