Fort with sheep inside - the smallest in number (6)
I believe the answer is:
fewest
'the smallest in number' is the definition.
'fewest' can be an answer for 'number' (I have seen 'Smallest number' mean 'fewest' so perhaps 'number' could also mean 'fewest'). I am unsure of the 'the smallest in' bit.
'fort with sheep' is the wordplay.
'fort' becomes 'ft' (abbreviation in place names).
'with' is an insertion indicator.
'sheep' becomes 'ewes' (a ewe is a female sheep).
'ft' enclosing 'ewes' is 'FEWEST'.
'inside' is the link.
Can you help me to learn more?
(Other definitions for fewest that I've seen before include "Smallest number" , "The least number" , "Minimum number" , "The lowest number" , "Least abundant" .)