The most common and most likely answer to this crossword clue is the 6 letter word SUCKLE
S
U
C
K
L
E
Buddy explains!
The straight part
We know that "Give sustenance to" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case give sustenance to = suckle.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 SUCKLE mean?
SUCKLE noun- A teat.
- To give suck to; to nurse at the breast, udder, or dugs.
- To nurse; to suck milk from a nursing mother.
- To nurse from (a breast, nursing mother, etc.).