The most common and most likely answer to this crossword clue is the 15 letter word WHATCHAMACALLIT
I've seen this crossword clue in Independent
W
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More clues leading to the result WHATCHAMACALLIT
- Opening in the morning: a visit inside comedian’s widget
- Women breed a tailless parrot with lines on it? I forget its name
- Greeting Davina, some might say it’s unmemorable thing
- Annoyed with a cat – all about cold meat, whatever it is
- Unnamed person has to develop a raincoat wanted by everyone in Intelligence
- Audibly witness a parrot under the influence producing something unmemorable
Buddy explains!

The straight part
We know that "I forget its name" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case i forget its name = whatchamacallit.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does WHATCHAMACALLIT mean?
WHATCHAMACALLIT noun- A metasyntactic term used for any object whose actual name the speaker does not know or cannot remember; a doodad, gizmo, thingamajig, thingy.