The most common and most likely answer to this crossword clue is the 14 letter word WHATS-HIS-NAME
I've seen this crossword clue in The Times
W
H
A
T
S
-
H
I
S
-
N
A
M
E
Buddy explains!
The straight part
We know that "I can’t remember who" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case i can’t remember who = whats-his-name.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.