Note that it's small at the bottom end (5)
I believe the answer is:
minim
'note' is the definition.
(type of musical note)
'it's small at the bottom end' is the wordplay.
'it's small' becomes 'mini' (short for miniature).
'at' says to put letters next to each other.
'end' says to take the final letters.
The last letter of 'bottom' is 'm'.
'mini'+'m'='MINIM'
'that' acts as a link.
(Other definitions for minim that I've seen before include "A note in music, half as long as a semi-breve- it's a palindrome" , "Note of two crotchets" , "Beat" , "Quarter of a breve" , "Half-note" .)