I have to follow frequency and number (4)
I believe the answer is:
five
'number' is the definition.
(five is a number)
'i have to follow frequency' is the wordplay.
'i have' becomes 'I've' (contraction of 'I have').
'to follow' means one lot of letters go next to another (some letters go after or follow others).
'frequency' becomes 'F' (common abbreviation as in FM, frequency modulation).
'ive' put after 'f' is 'FIVE'.
'and' is the link.
(Other definitions for five that I've seen before include "Prime number" , "Cube root of 125" , "V as a numeral" , "taken for the rest of the musicians" , "Quintet" .)