Narrow bit fits in top (7)
I believe the answer is:
limited
'narrow' is the definition.
(similar in meaning)
'bit fits in top' is the wordplay.
'bit' becomes 'mite' (I've seen this in another clue).
'fits in' means one lot of letters goes inside another.
'top' becomes 'lid' (lid is a kind of top).
'mite' going into 'lid' is 'LIMITED'.
(Other definitions for limited that I've seen before include "Narrow" , "Restricted in extent/size" , "Checked" , "Restricted in extent" , "Constrained" .)