In college, Heather meets posh boy speaking only one tongue (10)

'speaking only one tongue' is the definition.

'in college heather meets posh boy' is the wordplay.
I cannot quite understand how this works, but
'in' is within the answer.
'college' could be 'uni' (I've seen this in other clues) and 'uni' is located in the answer.
'posh' could be 'u' (abbreviation for upper-class - opposite to non-u) and 'u' is found in the answer.
'boy' could be 'gal' and 'gal' is found within the remaining letters.
A single letter 'l' remains which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?