Crossword Genius

Perhaps seventh word in a limerick's too long for it (7)

Ross

I believe the answer is:

ordinal

I'm a little stuck... Click here to teach me more about this clue!

'perhaps seventh' is the definition.
'ordinal' can be an answer for 'seventh' (similar in meaning). I am unsure of the 'perhaps' bit.

'word in a limerick's too long for it' is the wordplay.
'ORDINAL' is hidden within the letters.
But, I'm uncertain how the hidden word is indicated.

Can you help me to learn more?

(Other definitions for ordinal that I've seen before include "Denoting a numerical order in a series" , "Maybe first" , "First, for One" , "Numeral indicating rank order" , "gives position in class" .)

I've seen this clue in The Independent.
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