The most common and most likely answer to this crossword clue is the 5 letter word NATCH
N
A
T
C
H
Buddy explains!
The straight part
We know that "Certainly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case certainly = natch.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 NATCH mean?
NATCH adverb- Naturally; of course.
- The rump of beef, especially the lower and back part of the rump.
- A notch.