The most common and most likely answer to this crossword clue is the 6 letter word MEEKLY
I've seen this crossword clue in The Guardian
M
E
E
K
L
Y
More clues leading to the result MEEKLY
Buddy explains!

The straight part
We know that "lacking oomph" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lacking oomph = meekly.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 MEEKLY mean?
MEEKLY adverb- In a meek manner; quietly and humbly.