The most common and most likely answer to this crossword clue is the 7 letter word ENCORE!
I've seen this crossword clue in Financial Times
E
N
C
O
R
E
!
Buddy explains!
The straight part
We know that "A call for repeated effort" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a call for repeated effort = encore!.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 ENCORE! mean?
ENCORE! noun- A brief extra performance, done after the main performance is complete.
- A call or demand (as by continued applause) for a repeat performance.
- To call for an extra performance or repetition of, or by.
- To call for an encore.
- To perform an encore.
- (said by audience members after a performance) Please perform again!