Some letters from a lad over in Texas mission (5)
I believe the answer is:
alamo
'texas mission' is the definition.
(the Alamo Mission, fortress in Texas)
'some letters from a lad over' is the wordplay.
'some' says the answer is hidden in the clue (some parts of the word are used).
'over' shows that the letters should be reversed in order.
'omala' is hidden within 'letters from a lad'.
'omala' back-to-front is 'ALAMO'.
'in' is the link.
(Other definitions for alamo that I've seen before include "The --, Texan mission besieged in 1836" , "Texas fort famed for defence against Mexican army" , "US mission" , "The red planet" , "Where Jim Bowie died" .)