This is not an uncommon situation. A segment field may be defined as a basic non-composite type such as "ST" for a string field. However in fact for a given implementation the same field may actually be a composite field. In the message browser this field would look something like this when field numbering is switched on:

Wrong Composite Definition

Notice how the solution is easy. In the case shown it is just a matter of editing the segment definition and changing the Data Type of field 21 from ST to the actual composite data type. If the composite type does not exist then one can easily create a custom composite data type with Chameleon.