Left mate on roundabout, playing with a slide? (10)
I believe the answer is:
'a slide?' is the definition.
(I've seen this before)
'left mate on roundabout' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'roundabout' indicates an anagram.
'mateon' is an anagram of 'amento'.
'playing with' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for portamento that I've seen before include "glide -- in cantor's style?" , "Style of singing" , "sliding movement?" , "slide" , "smooth transition" .)