Two segments of a small role (3,4)
I believe the answer is:
bit part
'a small role' is the definition.
(I've seen this in another clue)
'two segments' is the wordplay.
'two segments' can mean two replacements for 'segment'.
'segment' becomes 'bit' (I've seen this before).
'segment' becomes 'part' (I've seen this in another clue).
'bit'+'part'='BIT PART'
'of' acts as a link.
(Other definitions for bit part that I've seen before include "'Small role in film or play (3,4)'" , "Tool" , "extra work, perhaps" , "cameo" , "Very minor role" .)