The most common and most likely answer to this crossword clue is the 11 plus 4 letter word  CONVERGING LENS
I've seen this crossword clue in The Guardian
Â
C
O
N
V
E
R
G
I
N
G
L
E
N
S
Buddy explains!
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.