Boatman under back of big empty llama-house where roof ends (5)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

A charade of G (‘back of bi G ‘) plus AB (able-bodied seaman, ‘boatman’) plus LE (’empty L lama-hous E ‘). Note the capital first word, to hide the lower case usage, and the hyphen in llama-house to get the correct emptying.