Top aide recalled number for fixed line (4)
I believe the answer is:
axis
'fixed line' is the definition.
(I've seen this before)
'top aide recalled number' is the wordplay.
'top' indicates taking the first letters (in a down clue, the first letter is top).
'recalled' is a reversal indicator (to recall can mean to bring something back).
'number' becomes 'six' (six is a kind of whole number).
The initial letter of 'aide' is 'a'.
'six' backwards is 'xis'.
'a'+'xis'='AXIS'
'for' acts as a link.
(Other definitions for axis that I've seen before include "Spindle - German/Italian alliance - deer" , "One of very elliptical orbit" , "Centre around which something rotates" , "vertebra" , "Line around which something rotates" .)