Line about number turning up close to fantastic US city (7)
I believe the answer is:
concord
'us city' is the definition.
(capital of New Hampshire)
'line about number turning up close to fantastic' is the wordplay.
'line' becomes 'cord' (cord is a kind of line).
'about' indicates putting letters inside.
'number' becomes 'n' (n can represent any number).
'turning up' shows that the letters should be reversed in order.
'close' suggests the final letters (I've seen 'close of' mean this (the letter which ends or closes the word)).
The final letters of 'to fantastic' are 'oc'.
'oc' written backwards gives 'co'.
'n'+'co'='nco'
'cord' placed around 'nco' is 'CONCORD'.
(Other definitions for concord that I've seen before include "Amity" , "Harmony; treaty" , "Harmonious state of things" , "Agreement, harmony" .)