Small firm periodically repair old sign (7)
I believe the answer is:
scorpio
'sign' is the definition.
(sign of the zodiac)
'small firm periodically repair old' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'firm' becomes 'co' (a firm is a company).
'periodically' indicates alternate letters (letters must be selected 'every so often').
'old' becomes 'o' (common abbreviation eg in OE for Old English).
The alternating letters of 'repair' are 'rpi'.
's'+'co'+'rpi'+'o'='SCORPIO'
(Other definitions for scorpio that I've seen before include "A heavenly body" , "Star sign" , "7 [SIGN OF THE ZODIAC]" , "Oct-Nov zodiac sign" , "Zodiac sign like stinging creature" .)