The most common and most likely answer to this crossword clue is the 7 letter word ENDGAME
I've seen this crossword clue in The Times
E
N
D
G
A
M
E
More clues leading to the result ENDGAME
- When a king could be under pressure to kill pheasants, perhaps
- Situation in which bishops may have been done away with
- Kill grouse etc, when ultimate strategy comes into play
- Sound final whistle, perhaps, leading to drama
- Men aged badly; this is the final stage
- Willing to finish first — it’s nearly over now
- Aim to gamble in contest’s final stage
- Point at Lord Hall gatecrashing celebrity finale
- Close match leading to drama
- Den rebuilt by wild animals in stage before mating, often
Buddy explains!

The straight part
We know that "when ultimate strategy comes into play" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case when ultimate strategy comes into play = endgame.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 ENDGAME mean?
ENDGAME noun- The final stage of a game of chess, when there are few pieces left.
- The final stage of a game of bridge, when there are few cards left.
- The final stage of an extended process or course of events, especially with the implication of the imminent realization of a masterful strategy or plan.
- The gameplay available in a massively multiplayer online role-playing game for players who have completed all of the preset challenges.